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## Water in the Cup

 Category: Science Submitted By: koma Fun: (3.34) Difficulty: (2.55)

A man in a restaurant asked a waiter for a juice glass, a dinner plate, water, a match, and a lemon wedge. The man poured enough water onto the plate to cover it.
"If you can get the water on the plate into this glass without touching or moving this plate, I will give you \$100," the man said. "You can use the match and lemon to do this."
A few minutes later, the waiter walked away with \$100 in his pocket. How did the waiter get the water into the glass?

### Answer:

First, the waiter stuck the match into the lemon wedge, so that it would stand straight. Then he lit the match, and put it in the middle of the plate with the lemon. Then, he placed the glass upside-down over the match. As the flame used up the oxygen in the glass, it created a small vacuum, which sucked in the water through the space between the glass and the plate. Thus, the waiter got the water into the glass without touching or moving the plate.
You can try this experiment at home with appropriate supervision.

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Comments on this teaser

 Posted by GebbieRose 11/11/06 We don't get nearly enough science teasers. Thanks for a good one! :D Posted by MalcolmReynolds 11/11/06 Wouldn't the flame burn down to the water before it used up all the oxygen? The plate was full. Posted by lifeisgood812 11/11/06 Sounds Cool! Posted by reptile5000 11/12/06 he could have either.... asked some1 else to do it, used gloves, tipped the table a little and put the cup to the side, etc. ...without touching it Posted by CountdownCrispy 11/12/06 Fun one! :D Posted by Tiberius 11/12/06 Actually, as the flame burns and uses up the oxygen, it replaces the gas with an equivilent volume of combustion products. The reason it creates a partial vacuum is that the flame heats the air, causing it to expand. This pushes some of the air out from under the glass. Then, once the oxygen has been used up, the flame goes out, and the air begins to cool. As it cools, the air contracts, causing the partial vacuum. Posted by doodinthemood 11/14/06 The difference in volume of warm air and cold air is very small. Only a tiny amount of the water would be sucked into the glass. Posted by kcheer2493 11/14/06 my science teacher, Mr. Poupolo, would apprecitate this. We're studying these kind of things Posted by teen_wiz 11/22/06 Thanks! That was AWESOME! :D Posted by bbbz 11/25/06 tiberious is correct. the water slowly rises into the glass after the match goes out and the air begins to cool. if it had anything to do with burning the o2, the water would rise immediately. doodinthemood makes a good piont as well. while the water would certainly rise into the glass, it is doubtful that you could get all the water to draw up. Posted by qwertyopiusa 11/29/06 koma: nice problem! I have done this experiment (with a candle) when I was a kid, so I knew the solution, but the lemon wedge got me thinking for a long while... :) Tiberius: your reasoning is ok, but when you do the experiment there are no bubbles coming out the glass. In addition, the water rises immediately after the flame dies... there is no time to cool the air just by conduction. I googled some data for air (dens=1.2kg/m3, spec.heat=1.005kJ/kg/K, expan.coef=3.43e-3K^-1, candela=18.4mW) and considering that the flame dies in 5 seconds and a typical glass of height=10cm and diam=5cm, the difference in volume might rise the water 0.13mm... when you do the experiment the water rises 150 times more. bbbz: you forgot the pressure drop at the border of the glass! It is necessary to build up some vacuum in the glass before the water can go through the tiny gap between the glass and the plate. MalcolmReynolds: very sharp! :) ...but koma never said that the plate was "full", he said the plate was "covered". Actually, you cannot put into the glass more water than 21% of its volume (the amount of oxigen you have burnt), so the flame will die by asphyxia before getting the opportinity to drown. :D Posted by stil 12/05/06 qwertyopiusa and others; I'll cast most of my vote to Tiberius. If "there are no bubbles coming out the glass," it is because the vacuum is formed much earlier. The visual aura/flame is accompanied by a significant volume of low density air (vacuum.) By the time a seal between glass and water is achieved, the vacuum is already there [to be trapped like a butterfly in a net] and the air pressure outside the glass is working against it. Contraction has more to do with being overwhelmed than with being cooled. One "ultimate" experiment involves a candle in a bowl. Wrap a toy rocket ignitor wire (wired to an ignitor) around the wick. Cover the candle with a glass. Letting a bent straw with one end inside the glass serve as a vent, add water to the bowl until the bottom one-fourth or one-third of the candle is under water. Remove the straw and flip the switch on the ignitor. Posted by scallio 12/12/06 Cool! 8) Can't wait to try this experiment with the kids. :D Posted by teddybair 12/12/06 I'm with scallio ~ my kids will love this one! Posted by cloughme 12/12/06 Cool! Posted by lmurray 12/12/06 A LOT OF BANTER ABOUT A VERY BASIC ELEMENTARY SCHOOL SCIENCE CLASS EXPERIMENT. IT WORKS AS THE MAN SAID. MAYBE SOME OF YOU WERE ABSENT THAT DAY. EASY, BUT I HOPE THE KIDS ENJOYED IT. :) Posted by HScott13 12/12/06 I'm with Tiberius, bbbz and stil on this one. The Oxygen is replaced by an equal volume of Carbon dioxide, therefore that doesn't cause the vacuum. The air has already expanded by the time you get the glass over the flame, thus no bubbles coming out. Use a bigger bottle (2 litre/4 pint works) and enough of the air is still cold at the point of covering that you do get the bubbles coming out as the air expands. This way, the flame lasts a lot longer (larger volume = more oxygen)and you see the water starts to be sucked in only as the flame is dying and the air cooling. Posted by marschie 12/12/06 The lemon wedge had me thinking about acidity -- even though I knew about the vacuum and all that. I'm easily distracted. :oops: Posted by puttumup 12/12/06 :o that was a good one it took me a couple ..i cant believe all the smack comeing out of you peaple... :evil: cant you just say something positive about the teaser ..without bieng so negitive and haveing to tare it apart :o Posted by coachpisco 12/12/06 Interesting.. Posted by bradon182001 12/12/06 I must have been absent that day as I had no clue. I'll have to take everyone's word for it that it works. Good piece of knowledge I'll try to remember next time it comes up in conversation (or is featured again on Daily Brain Teaser). :o Posted by Pizzazz2u 12/12/06 Let us just realize, this is a science teaser, and it does work! :P 8) I know because of my science friends and my science and chemistry classes. Awesome teaser! :wink: We need more of these to challenge us, daily! 8) Posted by kman613 12/12/06 A classic bar trick I've been doing for years to get free drinks - it always works! Posted by locutus13ds 12/12/06 thanks Mr. Wizard! I never knew that! 8) Posted by phyllisa 12/12/06 This sounds like fun. I never did this in Science class but it's never too late :) Posted by OldChinaHand 12/12/06 Nice One. Thanks for sharing it. 8) Posted by wordmama 12/12/06 I must have been 'absent that day' with Bradon! I am the worst science student; I'm a language person, so I didn't appreciate this one. However, I'm always saying 'something for everyone' and it was nice to see the interest shown in the responses, even if I myself had no idea what they were talking about! :o Posted by Punk_Rocker 12/12/06 Haha wow, we just did this in physics today only with testtubes and what not... Posted by Lady_Lani 12/12/06 interesting and educational. I love it 8) Posted by iqhigh 12/12/06 :) nice, educational & fun. good work! Posted by Badger 12/12/06 Fun, Koma. Thanks for submitting a basic illustration of Charles' Law of Gases in action. PS: I liked the feedback from the physicists out there! :D Posted by Infinitoid 12/12/06 thats pretty tight. ill be sure to try that one Posted by saradove 12/12/06 very interesting teaser i'm not good at these kinds but i luv 'em anyway!!!!! Posted by breathesunshine 12/12/06 Great one for the kids to be "wowed" by science! 8) Posted by becky18567 12/12/06 cool!!!! :D Posted by jabdr 12/12/06 My perfect record is intact ... Science teasers ..150 - jabdr... 0. :roll: 8) :D Posted by I_Write_Books 12/12/06 Fun, but easy for anyone who has taken basic chemistry! Posted by Aelcyx 12/15/06 qwertyopiusa -- You are forgetting how hot the flame is and that the density of the air changes greatly around it. To prove this, repeat the experiment by putting something hot inside that doesn't involve a flame (maybe one of those camping heat packs where you break it in half, or a coin that has been on the stove for a while (be careful when handling this -- use tongs if you are brave enough to do it)). I let the water sit for a while until no more water could come in and no air bubbles came in. Then I put an ice pack on top and bubbles started to re-form. This makes me think it has to do with air density and not the combustion reaction. Posted by 4demo 01/12/07 Good teaser overall, but I think you should add a simple detail: the waiter had a matchbox or some kind of lighter to light the match. I guess you could assume the waiter is allowed to light it, but it can't hurt. :) Posted by adiddie48 01/17/07 that is abs. amazing Posted by bgil7604 01/24/07 The only thing I can say about this is that I can not believe I actually got it right! This was a cool teaser. Posted by Blademaster 02/25/07 LOL!!!!! :D IT WORKS! Posted by LeafFan4life 02/26/07 this teaser isnt specific enuf i thot it meant nothing can touch the plate Posted by npf2005 03/04/07 I was just doing that at school in science. :o Posted by zips5000 03/23/07 i got it .. good one. Posted by brainglewashed 07/22/07 THAT WAS MY SECOND GUESS! :D jk Posted by brainglewashed 07/22/07 THAT WAS MY SECOND GUESS! :D jk Posted by Xosyaraiha 08/04/07 I'm very skeptical on whether or not this would work. If I had to bet \$100 on whether or not this would work, I would bet that it wouldn't. I mean, seriously, think about it, how does a match that will blow out in an instant after being deprived of oxygen create ENOUGH of a vacuum to suck up that much water??? And even if it did, as soon as the match blew out, the water would be all over the plate again. i'm sorry, it just doesn't make sense. pm me if you can explain. it's a good teaser, i'm just confused :-? Posted by aerdramaqueen 08/21/07 It really does work, and it looks really cool too! :D We did it science class! Posted by Fire_Shell96 11/21/07 u know, this can also fit under trick. slurp the water (without moving it) and spit it in the cup, although it might cause some water loss. lol, good teaser. Posted by cazoodles 11/22/07 cool experiment :) Posted by amymarie1123 01/08/08 hey everyone just did this experiment, it works! i used a candle instead Posted by jimmie 02/12/08 This is High School chemistry. The ideal gas states PV = nRT, pressure, P, times volume, V, equals the number of moles of the gas, n, times the gas constant, R, times the temperature, T. (Note one mole of an entity is 6.02 times 10 to the 23rd power instances of the entity) The air in the immediate vicinity of the flame is heated to approximately 451 degrees Fahrenheit and before the glass is placed over it is not constrained to a particular volume and is at the pressure of the surrounding atmosphere. As a result the air in the vicinity of the flame contains fewer moles of gas. Once the glass is placed over the flame the flame becomes oxygen deprived and begins to cool and when it is finally oxygen starved it is extinguished at which time a rapid cooling of the air occurs. Since the cooling gas occupies less space at the same pressure the atmospheric pressure pushes the water into the glass or the relative vacuum of the fewer gas molecule sucks up the water or the lesser volume occupied by that number of molecules at that temperature and pressure draws in the water, however you want to think about it. It is the cooling of the gas that creates the pressure differential that moves the water into the glass. Posted by drat 06/09/08 absolutly brilliant Posted by lwood5908 06/23/08 awesome teaser. I didn't get it right but it was fun. Posted by Sand_Kirton 07/20/08 I think this one is definitely more engaging for those of us who had NOT observed this experiment in science class. Stumped, I initially guessed that the waiter put the glass upside down over the lemon wedge at the center of the plate, using the match to prop up the edge of the glass so that water can flow in via osmosis. I'm dubious whether that would even work and certain it couldn't happen within a few minutes. Non-scientific solution? (The man said "you can" use the match and lemon; not that you MUST.) Its a restaurant so there must be a clean, dry napkin on the table. Drop that baby onto the plate to soak up all the water then squeeze out the napkin into the glass! LOL! Posted by scienth 09/05/08 The candle flame initially heats the air, which expands, increasing the air pressure in the glass. When the candle goes out the heat source is gone and the air cools and contracts, causing the pressure in the glass to drop and the pressure of the air outside the glass pushes the water into the glass. Posted by Cindzey 01/01/09 so THATs why my soda can exploded! Posted by tuffysos0915 02/15/09 I had this one at school, it was that same question. No one got it, ut now i have the answer! :P :lol: :evil: Posted by Paladin 03/16/09 To my fellow science geeks here who claimed that the O2 was replaced by an equal volume of CO2, you have either not done this experiment or are forgetting a crucial element. When the match is burned, it is oxidized, and the O2 combines with the body of the match to form a carbon residue (char) on the wick. The resulting reaction results in a much smaller volume of gas inside the glass, resulting in an instant vacuum that draws water up into the glass. I have done this experiment slightly differently using a small candle (a birthday cake candle) floating on a cork in a pool of water. Placing a closed container over the floating cork IMMEDIATELY draws water up into the glass until all the oxygen is consumed. There is no cooling of air or any amount of time for gas volume due to temperature differences to have much impact on the experiment... it is purely a result of the consumption of oxygen, which takes about 3 seconds or less (depending on the side of the glass). It is a very fun experiment and you should all try it. Posted by mathisnice 08/12/09 Good one! Posted by glowtmickey 12/12/09 This was my first Braingle Teaser. This is the teaser that lured me to braingle. Thank you so much for writing a good teaser that brought me to one of my favorite sites. Posted by isaihtb 12/12/09 I thought the waiter already had 100 dollars in his pocket when he served the plate so he didnt take the bet Posted by craniac 12/12/09 All this talk from people who haven;t actually done the experiment! It reminds me of a primary rule of science that a professor roared at me one day: G-- damn it, speculation is not a valid method for determining questions of fact! It can help you come up with a theory, but the only way to test a theory is to actually do a well-designed experiment and see what really happens. Posted by auntiesis 12/12/09 Cool teaser, it got a lot of people talking. I knew the basic principle, but got sidetracked by the lemon. :D :D :D Posted by Jake10 12/12/09 I didnt know you could do that!! :o Posted by UptheHill 12/12/09 YES, JOHNNY YOU CAN!!! 8) Posted by mepapa 12/12/09 ummm... I hope it was a strike anywhere match or he is out of luck :lol: :wink: :lol: Posted by elentir 12/14/09 @ Paladin - you are correct in that the water does start to rise immediately upon covering the flame. While that initial effect may be due to the consumption of O2 - it is slight and doesn't account for the majority of the fluid level rise, which begins as soon as the flame goes out. This effect is primarily due to the cooling of the remaining gas and the vacuum that creates. It is a demonstration of Charles' Law. Posted by Albert_Holmes 03/14/10 ....stupid man... ....did he think waiters are *THAT* dumb? ....i guess he's *MORE DUMBERER*... :lol: Posted by coolcat101202 07/22/11 We did this experiment once when i was a school and even though it was a long time ago i still remember it so i got it straight away. It's still a fantastic teaser though and you learn something at the same time, good work. Posted by plenum 12/09/12 Hold the glass over the lit match on the lemon wedge for a few moments BEFORE lowering the glass onto the plate with the water. This fills the glass with hot air better than immediately putting the glass over it, this providing a better vacuum. Posted by Babe 12/12/12 I have heard this one before, but being a waitess one time, I would have gone and got a straw and sucked up the water and emptied it into the glass. Problem solved! No need for a lemon and match :D Posted by HABS2933 12/12/12 Pretty easy for anyone who has ever taken a fun with science class (the U here runs them at the mall occasionally for the kids) or has seen Bill Ny Posted by cutebug 12/12/12 Been there done that, but a great teaser if you've never seen it done. :D :D Posted by eamon 12/12/12 The waiter could also just ask a friend to pour it in for him and split the money. The man said only HE couldn't touch the plate. Posted by iggy39 12/12/12 I loved babes comment. :lol: :lol: :lol: Posted by wmcleod 06/19/14 Bunk think about it... a lit match turns the solid in the match into a gas (smoke). This gas takes up much more space than the solid did. Instead, the glass might fill with warm air and rapidly cool, sucking a small amount of water into it once the flame was extinguished. Anyone doing this experiment would notice that a lit match would give no suction Posted by ultimatecub 02/27/15 Sorry, but this will not work. Only 21% of air is oxygen, the rest is mainly nitrogen, so it couldn't be a vacuum. Good idea though.

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