All living things are made up of cells. This concept, called the cell theory, was discovered by two German biologists, Theodor Schwann and Matthias Schleiden, and was compiled in a book by Rudolf Virchow.
In the 17th century, Robert Hooke sliced up a piece of cork and examined the pieces in a microscope. He observed objects that looked like little jail cells, and he named these objects "cells."
Hooke's observations paved the way for the development of the cell theory, which states that cells are the building blocks of all life, making up the structure of organisms and performing all the functions within organisms. Another part of the cell theory discusses that cells can only arise from pre-existing cells.