Cells are the basic unit of life. Plants and animals are made up of cells. In fact, the average human is made up of more than 75 trillion cells! These cells are so small they must be viewed with a microscope. Cells have different functions, and they are made up of even smaller parts, each of which is assigned a specific task. For more information on cells, read Cell Scientists: From Hooke to Nusslein-Volhard, another book in the Mission: Science series.