When to encourage writing on walls

I'm not sure what that, um, shape is supposed to be. But I do know that I won't be washing this off any time soon. The dried up boogies to the left of it are another story ...

P.S. We have a winner of Home Improvement: The Complete 7th Season! Mel, please send me your address and I'll get it in the mail. Thanks to everyone for the great Christmas-buying tips!

P.P.S. It was on his bedroom wall. I'd never allow boogers on the family room wall.