Your hand is a fridge. Encourage a friend to open the fridge, and browse the many tiny and invisible bottles of milk. Gently goad them into removing one of the bottles of milk, and drinking it. When they do, look knowing and snigger, eventually telling them that they chose the bottle that you'd weed into. When I did this, the "joker" would put their hand over their face in absolute horror at the prospect, or perhaps because they had just said "wee". This leads me to believe we were quite young. Be careful, though - if they haven't mimed a swallow, they could spit your invisible wee back into your face.

"Me chinese, Me play joke, Me do wee wee in your coke" Similar game. Punchline was, however, the above rhyme. The additional humour here is derived from the general knowledge that chinese people spend the majority of their time weeing into each other's soft drinks.