One day, the teacher asks a boy, "Why can't fish talk underwater?"
The kid says, "If I put your head underwater, will you be able to talk?"
Underwater Talking
Explanation
Explain Bear
Okay, so basically, the teacher asks why fish can't talk under the water. But the kid is a smart aleck and asks the teacher if *they* could talk underwater. It's funny because, duh, nobody can talk underwater, not even teachers, and certainly not YOU!