Nitrogen triiodide is one chemical you don't mess with, mainly because it requires very little stimulation to blow up.
It's so sensitive that something as small as a mosquito can set off a series of explosions.
The reason for its volatile nature is its chemical structure -- one nitrogen atom surrounded by three iodine atoms.
They say three is a crowd and in the case of this molecule, the iodine atoms only crowd around one part of the nitrogen atom. This causes a bond strain making the entire molecule quite unstable.
What happens as a result is simple -- the molecule falls apart and recombines to form an entirely different structure.
Not only are the series of reactions fast but they are also quite beautiful, with the three iodine atoms giving off a beautiful purple gas.