It's more or less the force of the impact, along with the suprise of it happening. Also the bit where IT STINGS LIKE A UNHOLY GIANT BEE STUNG YOU IN THE FACE WITH A LONG SWORD.
Also, I have one of those guns on key-chains, though it is grey. Bought for for 5 Euros, loving French stuff ripped me off.
I duno, it doesn't hurt THAT much.
Whip it around in circles until it hits someone/something. c:
when i read that, i just burst out laughing, you said it in such an innocent way.