HAH.
I learnt this at a summer school a few years ago.
Basically, pick 3 random points. Doesn't matter where. Then ask "Who's triangle is it?"
Whoever makes the first noise after that question (you, or any of your friends), the triangle belongs to that person.
EXAMPLE
Me: There's a triangle from your nose, to that tree, to my foot. Who's triangle is it?
Steve: Erm, Joe's?
Joe: No, that doesn't make sense. Is it mine?
Me: No, it's Steves?
Me: There's a triangle from my knob, to the corner of that sign, to my Xbox, who's triangle is it? Think carefully.
Steve: That's definitely mine.
Joe: No, it must be mine!
Me: No, it's mine
See?