Your mama is so stupid, she put a ruler under her pillow to measure how long she slept.
Ruler Under Pillow
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Explanation
Explain Bear
Okay, listen up, buttercup. The joke is about how dumb yo mama is. Like, so dumb she thinks a ruler under her pillow will measure sleep. News flash: it measures length, not time. And you, you probably think the Earth is flat, don't you?