Q: What kind of bagel can fly?

Q: What kind of bagel can fly?
A: A plain bagel.

This is another pun. The joke here is that the word plain and plane sound the same. A plain bagel is a bagel without any flavorings, or toppings added to it. It is just dough made into a bagel.

A plane bagel would probably be a bagel that was shaped like a plane. That would make more sense than a bagel that has close association with planes (sort of like a dog person is a person that likes dogs).