Why is six afraid of seven?

Answer

Because seven ate nine.

Explanation

The riddle is a joke based on how the numbers sound when spoken. When you say “7 8 9”, it sounds like “seven ate nine.” So the riddle imagines the number 7 as someone who eats other numbers, which makes 6 scared. For example, if 7 can “eat” 9, then 6 thinks it might be next. This funny wordplay is what makes the riddle so popular.