One of Nature's wonders that you don't have to venture into the outofdoors to observe, the ability for the common housefly to walk across a smooth-surfaced ceiling without falling off, has parents groping for answers when children ask about the feat.
  A common response goes something like this: "Well, Jimmy, you see, houseflies have these tiny suction cups on their feet. They use these to cling to smooth surfaces like ceilings and window panes." The wise parent follows the explanation with a demonstration on how suction cups work so the child's attention gets diverted elsewhere — at least until the parent can study up on the matter and discover just how flies do manage the upside-down cling.
  For one thing, the suction cup explanation (read "myth") does not quite work. If flies suction-cupped their way across a pane of glass or ceiling, they would not have the ability to disappear in the nanosecond before a fly swatter descended upon them. The suction cups would hold them immobile a fatal second too long.
  The truth often turns out quite different from what people imagine. Like a lot of Earth's creatures, flies have claws. These tiny claws enable a fly to grasp just about any surface that has the least amount of roughness. To the human eye, the surface may appear as smooth as glass; to a fly, the surface provides many claw holds. Even the smoothest-appearing ceiling usually will have plenty of rough or raised areas for a fly to cling to.
  Some flies have additional means of clinging to seemingly slick or smooth surfaces. The housefly in particular has tiny pads with multiple fine hairs that produce a sticky goo which helps the fly cling to the smoothest surface of glass or metal or painted furniture or appliance.

(photo by blackbird at