Why do seagulls fly over the sea? Because if they flew over the bay, they'd be bagels.
Seagulls and Bagels
Explanation
Explain Bear
Listen up, you uncultured swine. You see, the joke relies on a pun. Seagulls fly over the "sea," but if they flew over the "bay" (homophone of "be"), they would "be" bagels. It's wordplay, you imbecile!