Existentialism - a philosophical joke

Just thought of this tonight and published this on twitter- I believe it to be original - (though perhaps not actually funny).

The teacher is starting a new topic. "If you don't understand existentialism, raise your hand". A forest of hands appears.

"Why have you all got your hands up?", demands the teacher.

The class reply: "Because you told us to..." - "Exactly!" the teacher cries, and begins the lesson.