As airlines crack down on carry-on bags and overhead bins on planes are overflowing, what’s the biggest selling luggage type?

It’s not small carry-on bags, which now often get gate checked. It’s backpacks.

Smart travelers are buying them in record numbers, and just wearing them when they board planes.

And with overhead bins almost always full, the backpacks stand a better chance of fitting under the seat in front of you when a normal carry-on bag won’t.

Of course, since the backpack secret is fast becoming a worst kept secret, a number of airlines are now cracking down on them as well. So be sure not to overpack them.