a phrase used by English teachers to signify that the person with the "great idea" will be receiving ice cream sandwiches from his classmates for his great idea
Joe: Are we having a test today?
Teacher: What a great idea! Hey everyone, did you hear that? Joe just had a great idea. Let's have a test today. Joe, do you like ice cream sandwiches, because everyone in here's going to be tossing them at you now.