when your reading harry potter out loud and everyone gets bored, so you spice it up by saying willy instead of wand everytime its mentioned :D
Harry took the willy. He felt a sudden warmth in his fingers. He raised the willy above his head, brought it swishing down through the dusty air and a stream of red and gold sparks shot from the end like a firework...
"I'm bored, shall we play The Wand/Willy Game?"
"Dude, best wand/willy game EVER!"