Joan Lawrence, age 13, of Fredericton, N.B., Canada, for her question:

WHAT EXACTLY ARE PROTOZOANS?

They were the very first animals that appeared on the planet earth  and they still outnumber all the other animals that arrived later. They teem everywhere, in soil, water and often in plants and larger animals. We do not notice the earth's protozoans because they are single cell creatures, too small for our eyes to see. Most of them are harmless to us, some are very useful and a few types cause diseases.