How do flys walk on ceilings?

Suction caps on their feet. To me the real magic is how they land upside down. I found the answer though some research. "On approaching the ceiling, and while still flying right side up, flies extend their forelegs over their heads till they can grab a landing spot with the suction caps in their feet. Their momentum then enables them to swing their hind legs up, like a gymnast on a trapeze."

However, MSNBC reported (related link below) that:

Scientists once thought that the curved shape of the hairs suggested that flies used them to grip onto the ceiling. In fact, the hairs produce a glue-like substance made of sugars and oils. A research team from the German Max Planck Institute for Metals Research recently studied more than 300 species of wall-climbing insects and watched them all leave behind sticky footprints. Team leader Stanislav Gorb presented the findings at the Annual Meeting of the Society for Experimental Biology in April.

It was found that aforementioned meeting was at University of Kent at Canterbury Monday 3rd - Friday 7th April 2006.