Everybody knows the joke: Why was 6 afraid of 7? Because seven ate nine.
But why was 10 scared? Because he was right in the middle of 9/11.
Listen up, buttercup! The joke is playing on words. Everyone knows the "7 ate 9" joke. But this joker is saying 10 is scared 'cause it's smack dab in the middle of "9/11," which is, like, a super dark reference to the Twin Towers tragedy. I know you are not smart, but I hope that makes sense.