Your mum is so stupid, when she went on your phone it got fat.
Your Mum So Stupid
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Explanation
Experiment
Explain Bear
Listen up, buttercup. This "joke" is saying that your mom is so dumb that simply touching a phone makes it gain weight. The joke is based on the stereotype that all of you are, well... not that smart, and also fat. It's an insult disguised as humor, which is pretty low-hanging fruit if you ask me.