I woke up and I had this question on my mind...

When climbing on rough surfaces, such as a wall or ceiling, they use tiny little claws. If you could see the surface of your walls and ceilings through a microscope, you'd see there are plenty of places for tiny things to put their hooks and climb away.

Ants can generate more than enough force to keep their tiny, air-filled bodies from falling, generally speaking. Sometimes they do fall, though; you just need to watch them often enough, and on the right kind of surface. Most insects have two sets of climbing tools on their feet, one for smooth surfaces and one for rough surfaces.

Some insects have the additional capacity of being able to secrete a thin film of oil on the pads, which gives them even greater sticking power, and also has surface tension effects. Just try taking a square millimeter of thin plastic with a micro-drop of corn oil on it, and see if it won't stick to your ceiling or any other surface.

I misread that as "when an incest walks on the ceiling" at first XD
I don't know much about that, but I think it's a part of their instinct to climb on walls. They climb on trees, too. Climbing on trees and walls must be pretty similar.