Ann Rainey, age 13, of Antes Fort, Pennsylvania, for her question:

Why does a bang on the head make a bump?

It is logical to expect a bang on the head to leave a dent. After all, a hard hit makes a dent in a melon and in most other materials. The fact is that a severe bash on the head may indeed make a dent    a serious shattering of the skull bones below the surface. Fortunately; a medium sized bash does not do this. The area swells up with a surprising bump instead.

A minor mishap of this sort actually creates a wound, even though it does not draw blood. Numerous tiny cells in the fleshy tissue are injured and disrupted and the body immediately sets about repairing the damage. The injured tissues need extra supplies of nutrients to rebuild themselves. Various body fluids rush to the disaster area with these supplies and float around to distribute them where they are needed. The extra fluids also help to hold the broken tissue in shape and to protect them from further collapse while the rebuilding is in progress. But they also create a swelling.