Home science experiments
Take some thermal printer paper -- I noticed this with a Cala Foods receipt, for example, which is printed on a thermal receipt paper. Make sure you haven't just washed your hands, and find a blank spot on the paper. Now press firmly to leave a fingerprint on the paper. You probably can't see your fingerprint. But now hold the paper carefully over a candle flame. Your fingerprint is revealed!
The oils in your fingerprint must either do something chemically to the paper or else prevent that part from getting quite as hot as the surrounding parts. It's like lemon juice, without the need for a lemon.
Thermal paper is getting much rarer; it used to be one of the most popular printing technologies, especially because it was pretty fast and quiet compared to things like daisy-wheel or dot matrix printers. But now it's mostly been displaced by inkjet and laser technology.
Why was I holding a receipt over a candle flame, you ask? To destroy the credit card information there, of course. A candle flame "prints" on thermal paper with huge black splotches; if you're careful, you could write words by moving the paper quickly. Interestingly, but not too surprisingly, thermal paper doesn't catch fire as readily as regular paper. This feature is very important. :-)
Now, if only I could find that kind of plastic bag that Krazy Glue burns holes in...
