I may have mentioned this before...
One of the cute little things we were shown in Chemistry class was that if you dissolve iodine crystals in ammonia and paint the resultant stuff on a surface and let it dry, it becomes an extremely sensitive contact explosive. The best part is that it makes everything in the immediate vicinity purple.

We swiped the iodine crystals and anhydrous ammonia, made a fair bit of the stuff, then painted it on the undersides of the toilet seats in the girls' bathrooms.

During gym class we were easily able to identify our victims by their purple thighs...
( The Resident Loon Not a demographic. Do not measure., Thu 8 Sep 2011, 14:15, Reply)