Your forehead and your hairline must be old friends, because they go way back.
Receding Hairline
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Explanation
Experiment
Explain Bear
Listen up, buttercup. This joke is saying your hairline is receding so far back that it's like your forehead has been friends with your hairline for a super long time. Get it? It's 'cause it's so far back! You probably wouldn't know anything about that.