Invisible Ink
Science brain teasers require understanding of the physical or biological world and the laws that govern it.
There are two secret agents who want to send letters to one another but think that someone might try to read them. The first agent came up with an idea to send a letter to the other agent by writing with invisible ink. He then put the letter in his mailbox for delivery. When the other agent received the letter, he opened it and began to read its contents. How was it possible for the first agent to write with invisible ink and how was the other agent able to read it?
Answer
1) The first agent wrote the letter with the liquid of a squeezed lemon (citric acid). When the paper is dried nothing can be noticed on the letter.
2) The other agent held the letter over a flame and did not touch it. The hidden ink (juice) becomes a reddish-brown color and the words can be clearly seen.
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Comments
bluetwo 
Jan 14, 2003
| fun science project for kids! it also works with milk and vinegar, if you don't have lemon juice handy. you can also reveal the secret lemon message by spreading iodine or red cabbage juice over the paper. Also, a safer way than using a flame (read: flammable) is to hold the paper over a light bulb or to iron it. |
electronjohn 
Jan 14, 2003
| Bluetwo, 007 does not worry about the safety of using flames! lol Well you two have just about ruined the spy business by giving up one of there best kept secrets. Just kidding, I knew about this one since I was a kid. |
jonnyonline
Jan 17, 2003
| the spy could also write using white liquid paper and the other spy could hold that to a lightbulb too. |
Deucex2
Jan 17, 2003
| I highly doubt it jonny. As any college student can tell you, white-out leaves a very telltale mark and is VERY easily seen.
My favorite method for sending a sensitive message is on flash paper... once the person has seen the info they need, all they do is touch a match to it and FOOM it's gone in the blink of an eye. It's not invisible writing, but it's fun anyway. |
speedyg1000
Jan 18, 2003
| has somebody torn the"?" key off his keyboard? |
jonnyonline
Feb 18, 2003
| the message could be sent telepathically -- therefore making it invisible to the naked eye. |
Gander
Aug 01, 2004
| The agent opened the letter and began to read it, which would indicate to me that it was readable before using the heat lamp. Perhaps it sat in the mail box to long in the sun?
I hear you can see Cat urine with a black light.  |
vbguy101
Sep 02, 2006
| Duh!
Anyway, you could also use salaiva(spit). It's kind of nasty, but it does work. |
dudebomb93
Oct 29, 2007
| WOW I DIDTN KNOW THAT |
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