In 1665, Robert Hooke saw cells in a dead slice of cork. He though they looked like the cells in a monastery and the term stuck. In 1683, Anton van Leeuwenhoek looked at everything from rainwater to tears. These scientists and others developed the Cell Theory . Every living thing is made of cells.
Every cell comes from a pre-existing cell.
This sounds pretty simple to us, but it wasn't always understood. For a long time people believed in spontaneous generation . They believed flys came from rotting meat and frogs from mud. It took a couple hundred years to disprove those ideas and confirm that every cell comes from a pre-existing cell. From Tim Lynch with Kent State .