Your forehead is so big that the teachers used it as a whiteboard.
Giant Forehead
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Explanation
Experiment
Explain Bear
Listen up, buttercup. This joke is saying your forehead is HUGE, like so big the teachers can write on it like a whiteboard. It's an insult, but also kinda funny 'cause imagine having a fivehead that big! You'd need a whole cleaning crew to wipe it down after every class! This is a silly insult about your body.