Napalm

This is a personal favorite of mine. Napalm! To make it, get something to hold your mixture in that you're never going to use again for anything but making napalm. Either can go first, but you're going to be mixing styrofoam and gasoline. This doesn't take too long, so it's not a horrible idea for improv. However, it lasts quite a while, so consider making a batch every once in a while and throwing out your last batch. Anyway, mix the styrofoam and gasoline together until it's a sticky, syrup-like mess. When the mixture won't take anymore styrofoam, your product is complete. Be well aware, it's dangerous as hell.

Concept by Jolly Roger