How can a fly walk on the ceiling?

   A fly has a little suction cup on each of its six feet. This is how the suction cup works: It is hollow inside and slightly moist. When it is pushed against a flat surface, most of the air is squeezed out. The moisture seals the edges and keeps air from getting back in. Now there is a lot of air outside and very little inside the cup. The pressure of the outside air is strong. It holds the suction cup tightly to the ceiling.
   The fly picks up three of its feet at a time to step forward. The suction cups on the other three feet hold the fly in place till their turn to move. Does this seem like a hard way to walk? It would be hard for us, but not for a fly. Flies have great strength for their size.