Explain the three parts of cell theory and use a microscope (or a strong magnifier) to gather evidence that living things are made of cells.
Zoom into anything alive -- a leaf, a drop of pond water, the inside of your cheek -- far enough, and you hit the same building block every time: the cell. Cells are the basic unit of structure and function in every living thing. Some organisms, like bacteria and amoebas, are just one single cell doing every job alone. Others, like you, are made of tens of trillions of cells, each one small enough that a hundred of them could fit across the width of a human hair.
This idea took centuries to build. In 1665 the English scientist Robert Hooke looked at a thin slice of cork through a homemade microscope and saw tiny box-like compartments, which reminded him of the small rooms, or "cells," where monks lived -- so he named them cells, even though the cork cells he saw were empty dead walls, not living ones. Almost two hundred years later, after many scientists examined plant and animal tissue under improving microscopes, Schleiden, Schwann, and Virchow put the evidence together into cell theory, three ideas that still hold up today: (1) all living things are made of one or more cells, (2) the cell is the basic unit of structure and function in living things, and (3) all cells come from cells that already existed.
Mastery looks like: They can state all three parts of cell theory accurately and describe what they would expect to see examining any living tissue under magnification.
Common stumbles: Treating a microscope like it shows an unrelated new world rather than the same object enlarged; forgetting that ALL living things are covered by the theory, not just the ones they have personally viewed.