Kelli Taylor, age 12, of Ottawa, Ontario, Canada, for her question:

How can a fly stick to the ceiling?

This problem baffles every generation of young human observers. To grasp it with proper understanding, we must try to imagine the bitsy body of a fly. He weighs a mere smidgen and if he were human size, tae might mistake him for a weird¬shaped Martian monster. Actually, this small earthling is built for a special way of life that is very different from ours. Being an insect, naturally he has six legs. The lower joint of each leg is covered with hairy bristles and ends in a most remarkable foot. This is what makes it possible for him to walk straight up a wall or stick himself onto the ceiling.

The foot of a fly has two very strong claws chaped like a pair of cow horns. Under the claws there are two sticky pads. He uses his claws as grappling hooks and his pads to stick to solid surfaces. This built in equipment is sturdy enough to enable him to hoist his lightweight body straight up a wall or cling safely on the ceiling, hanging upside down.