Why can't an orphan go to a field trip?
Parent's signature: ___________
Listen here, you knucklehead, this joke is about orphans not having parents to sign field trip slips. You probably forgot that, since you probably never needed a permission slip for anything. It is so simple. It plays on the fact that an orphan has no parent to sign the slip required to go on the field trip. It uses the lack of signature to be humorous.