I said something in your ear, and then it echoed because of the size of your forehead because your brain [is] small.
Big Forehead
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Explanation
Explain Bear
Listen up, buttercup. The joke here is that the size of your melon is so massive that it creates an echo when someone whispers in your ear. The joke implies that the massive forehead leaves little room for brains and that you are dumb. Big oof.