The cell theory is an explanation of what cells are and why they exist. The three main parts are: 1) All living things are made of cells and their products, 2) New cells are created by old cells dividing into two, and 3) Cells are the basic building units of life. In 1665, Robert Hooke observed, with the aid of a crude compound microsope, the structure of a thin slice of cork. The structure resembled stacks of hat boxes, or holding cells, so he called what he observed "cells". In 1674, Anton von Leeuenhoek observed, in a sample of pond water, small animal-like creatures swimming around. He probably was looking at Spyrogira, but he called these small organisms "animal-cules". Theodore Schwann and Matthias Schleiden are responsible for formulating the first two parts of the cell theory, which was a scientific explanation of what these scientists had observed up to this point. A few years later, Rudolph Virchow added the third part of the cell theory, that all things are composed of these basic building blocks called cells.