Approach your victim and tell him that you will say to him the names of various birds, and after each one he must say 'Why?'.
You: 'Eagle'
Him: 'Why?'
You: 'Swan'
Him: 'Why?'
You: 'Sparrow'
Him: 'Why?'
You: 'Crow'
Him: 'Why?'
You: 'Duck'
Him: 'Why?'
You then swing your arm in a horizontal arc and smack him on the side of the head as hard as you can.
It should be a him, as girls lack the linguistic sophistication required for a joke of this cunning. Plus, they might say "Saying why after each bird makes no sense. I'm not going to indulge this folly," the divs.