Think of an animal.

Was the first thing you thought of a sponge? Probably not. They don't look much like animals, and unlike most animals, sponges don't move around once they're adults. Not surprisingly, for a long time people thought they were plants. After years of study, scientists realized that sponge cells are more like those of animals than plants.

The cells of most animals are organized into tissues and organs. Each cell has its special place and depends on the other cells in the organism for survival. Once again, sponge cells are a little different.