For a lot of parents, this occurs on almost a daily basis. Your child will say or do something that's wildly inappropriate, and your natural response to it is to laugh, but you can't. Laughing would tell the child that what they did is funny, and they shouldn't know that until they're older!
Unless you're raising a little angel, at some point you'll get a phone call like this one, and you'll be told that your child did something truly outrageous. Who among us didn't see someone at school moon the class during our education? You'll have to talk to them about it when you get home, but it's better to get all the laughter out of your system before you walk through the door. You have to set a good example, even if you know you would have cried laughing if you were there!