Microscopes were invented in the 1600s. Robert Hooke first looked at a thin slice of cork in 1665; he saw "a lot of little boxes." These little boxes first reminded of the little rooms monks lived in, so he called them cells. Hooke observed the same pattern in the stems and roots of carrots and other plants. What Hooke still did not know, however, was that cells are the basic units of living things.
Ten years later, the Dutch scientist Anton van Leeuwenhoek focused a microscope on what seemed to be clear pond water and discovered a wondrous world of living creatures. …
