Cells are the basic units that make up all living things, though they are too small to see without a microscope. Robert Hooke discovered cells in the 1600s while examining plant tissues under an early microscope and found they resembled small rooms, which he called "cells". Cells contain all the necessary materials for life and come together to form organisms like trees, birds, flowers, dogs and plants, while non-living things like computers, umbrellas and mugs do not contain cells.