6 looks like someone facing up.
9 looks like someone facing down.
69 looks like 2 people sucking each other's dicks.
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Explanation
Explain Bear
Listen up, you smooth-brained ape! The joke is all about how the numbers 6 and 9 resemble people in certain positions. Put 'em together, and you got a visual gag about a sex act. Get it now, or do I need to draw you a diagram with crayon?