Game with No Name, the
At primary school there was a boy called Tom who had orange wee. During toilet breaks, we'd line up at the trough-style urinal with Tom at one end and the rest of us at the other. The idea of the game was to repel Tom's orange wee with normal yellow piss for as long as possible. The game ended when Tom declared that his mum had taken him to see the doctor and he wouldn't be pissing orange any more.
written by St* Poly*arp*, approved by Phil

Here's another GWNN example, but not nearly as fun. Or, indeed, clever.
Kid A: What's your favourite animal?
Kid B: Leopard.
A: What's your favourite colour?
B: Green.
A: What's your favourite number?
B: Six.
A: I've never seen a green leopard with six legs!
Flawed, because as soon as someone had done it to you once, you could say dog, black, four to the next person, leaving them powerless to do anything but dither and say 'ermm...'
Not played beyond the age of 6.
written by excluded pupil, approved by Conor