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Water in the Cup

Science brain teasers require understanding of the physical or biological world and the laws that govern it.


Puzzle ID:#33929
Fun:**** (3.34)
Difficulty:*** (2.55)
Submitted By:komaAjp***




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?

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Nov 11, 2006

We don't get nearly enough science teasers. Thanks for a good one!
Nov 11, 2006

Wouldn't the flame burn down to the water before it used up all the oxygen? The plate was full.
Nov 11, 2006

Sounds Cool!
Nov 12, 2006

he could have either....
asked some1 else to do it,
used gloves,
tipped the table a little and put the cup to the side,
...without touching it
Nov 12, 2006

Fun one!
Nov 12, 2006

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.
Nov 14, 2006

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.
Nov 14, 2006

my science teacher, Mr. Poupolo, would apprecitate this. We're studying these kind of things
Nov 22, 2006

Thanks! That was AWESOME!
Nov 25, 2006

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.
Nov 29, 2006

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...

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.

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.

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.
Dec 05, 2006

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.
Dec 12, 2006


Can't wait to try this experiment with the kids.

Dec 12, 2006

I'm with scallio ~ my kids will love this one!
Dec 12, 2006

Dec 12, 2006




Dec 12, 2006

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.
Dec 12, 2006

The lemon wedge had me thinking about acidity -- even though I knew about the vacuum and all that. I'm easily distracted.
Dec 12, 2006

that was a good one it took me a couple ..i cant believe all the smack comeing out of you peaple... cant you just say something positive about the teaser
..without bieng so negitive and haveing to tare it apart
Dec 12, 2006

Dec 12, 2006

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).
Dec 12, 2006

Let us just realize, this is a science teaser, and it does work! I know because of my science friends and my science and chemistry classes. Awesome teaser! We need more of these to challenge us, daily!
Dec 12, 2006

A classic bar trick I've been doing for years to get free drinks - it always works!
Dec 12, 2006

thanks Mr. Wizard! I never knew that!
Dec 12, 2006

This sounds like fun. I never did this in Science class but it's never too late
Dec 12, 2006

Nice One. Thanks for sharing it.
Dec 12, 2006

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!
Dec 12, 2006

Haha wow, we just did this in physics today only with testtubes and what not...
Dec 12, 2006

interesting and educational. I love it
Dec 12, 2006

nice, educational & fun.
good work!
Dec 12, 2006

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!
Dec 12, 2006

thats pretty tight. ill be sure to try that one
Dec 12, 2006

very interesting teaser
i'm not good at these kinds but i luv 'em anyway!!!!!
Dec 12, 2006

Great one for the kids to be "wowed" by science!
Dec 12, 2006

Dec 12, 2006

My perfect record is intact ... Science teasers ..150 - jabdr... 0.
Dec 12, 2006

Fun, but easy for anyone who has taken basic chemistry!
(user deleted)
Dec 12, 2006

LOL!! I thought the match was to poke a hole in the lemon, suck the water through the lemon, and then spit it into the glass!
Dec 15, 2006

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.
Jan 12, 2007

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.
Jan 17, 2007

that is abs. amazing
Jan 24, 2007

The only thing I can say about this is that I can not believe I actually got it right! This was a cool teaser.
Feb 25, 2007

Feb 26, 2007

this teaser isnt specific enuf i thot it meant nothing can touch the plate
Mar 04, 2007

I was just doing that at school in science.
Mar 23, 2007

i got it .. good one.
Jul 22, 2007

Jul 22, 2007

Aug 04, 2007

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
Aug 21, 2007

It really does work, and it looks really cool too! We did it science class!
Nov 21, 2007

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.
Nov 22, 2007

cool experiment
Jan 08, 2008

hey everyone just did this experiment, it works! i used a candle instead
Feb 12, 2008

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.
Jun 09, 2008

absolutly brilliant
Jun 23, 2008

awesome teaser. I didn't get it right but it was fun.
Jul 20, 2008

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!
Sep 05, 2008

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.
Jan 01, 2009

so THATs why my soda can exploded!
Feb 15, 2009

I had this one at school, it was that same question. No one got it, ut now i have the answer!
Mar 16, 2009

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.
Aug 12, 2009

Good one!
Dec 12, 2009

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.
Dec 12, 2009

I thought the waiter already had 100 dollars in his pocket when he served the plate so he didnt take the bet
Dec 12, 2009

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.
Dec 12, 2009

Cool teaser, it got a lot of people talking. I knew the basic principle, but got sidetracked by the lemon.
Dec 12, 2009

I didnt know you could do that!!
Dec 12, 2009

Dec 12, 2009

ummm... I hope it was a strike anywhere match or he is out of luck
Dec 14, 2009

@ 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.
Mar 14, 2010

....stupid man...
....did he think waiters are *THAT* dumb?
....i guess he's *MORE DUMBERER*...
Jul 22, 2011

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.
(user deleted)
Aug 04, 2011

Seems to me like he touched it twice - once with the lemon and again with the cup. If skin contact is the rule he could have just used gloves!
Dec 09, 2012

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.
Dec 12, 2012

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
Dec 12, 2012

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
Dec 12, 2012

Been there done that, but a great teaser if you've never seen it done.
Dec 12, 2012

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.
Dec 12, 2012

I loved babes comment.
Jun 19, 2014


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
Feb 27, 2015

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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