Yo mama is like train tracks; she gets laid all around the country.
Train Tracks
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Explanation
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Explain Bear
Listen up, dummy! The joke is that "yo mama" is being compared to train tracks. Train tracks go all over the country, and people "lay" them, meaning they build them. The joke is that "yo mama" is sleeping around with a bunch of people all over the country. You probably have never been laid, LOL.