How can a fly walk on a ceiling?

Two sharp claws and sticky pad under each foot help the fly cling to ceilings and other smooth surfaces.  If you were to look at the feet of fly through a microscope, you would see that each foot is equipped with tiny claws and sticky pads of hair.

These enable the fly to walk upside down on the ceiling and cling to the slippery surfaces of windows and mirrors.  When walking upside down, the fly picks up three of the feet at a time, while the other feet hold the fly to the ceiling until it is their turn to step forward.

Flies have great strength for their size and can run along a ceiling with the greatest of ease. Most kinds of spiders and insects have claws on their feet that help them cling to ceilings and smooth walls.

Wherever the spider goes it lays down a thin, silken dragline, to help prevent falls or to escape from enemies.  If danger threatens, it can drop to the floor below, or it can simply hang there until the danger has passed.  

Then it climbs back up on its dragline and continues on its way.  Many of the duty cobwebs you see hanging from the ceiling are discarded draglines.-Dick Rogers