Before We Leave To Go Trick-Or-Treating

I just want to express my disappointment with Trevor.
I made a point to tell him over the phone last night, "Don't lose your Spiderman mask at school."
He said OK.
I call to tell him we're leaving in about a half an hour and I can't wait to take him trick-or-treating.
He said, "I lost my mask at school."

D'oh!