Changing colour
Squid ink risotto turns poo a beautiful charcoal colour, and when you wipe your bottom afterwards you get lovely black smears on the paper.

Oh, with respect to poo fingers in the event of not enough toilet paper- I lived for a while in Pakistan, where there is no vogue for toilet paper. You have a watering can and scoop the stuff away with your fingers. This is why it is considered impolite there to eat with your left hand- that's the one with poo fingers.
(I grew it myself, Sat 29 Mar 2008, 9:14, Reply)