You don't have to worry about running while boys are around. Even I can't see anything there.
Lack of Chest
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Explanation
Explain Bear
Alright, listen up, genius. The joke is implying that the person is so flat-chested that even the joke teller can't see anything there when boys are around and they're running. It's a burn, see? And BTW, I bet you can't even bench press your own weight, can you?