"Knock, knock."
"Who's there?"
"Not."
"Not who?"
"Not your dad."
Absent Dad
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Explanation
Experiment
Explain Bear
Hey, genius. The joke is funny because the dude on the other end says "Not" like it's a name. But then he hits you with the truth bomb: "Not your dad," meaning your dad is missing in action. It's dark humor for dummies, like you!