My 2nd year (age 12/13) science teacher made my mate & a randomly picked boy stand at the front of the class & then told everyone there was a physical attraction between them which increased as they moved closer.  He was obviously well ahead of the game on this one (she was very embarassed, I'm sure he could have used pens or teacups or something & saved her blushes).

Oooh, I should have been more clear, I meant he was ahead of the game on the whole physics/dating thing, I can't even remember who the boy was, but my mate didn't fancy him.