Some living things are a single cell, such as bacteria and yeast. Most living things we can see are made of many cells, and a human body holds trillions of them. Baking yeast is a living single-celled fungus: given warm water and sugar it releases a gas, carbon dioxide, which can inflate a balloon, while plain water does nothing. Individual cells are too small to see without a microscope, and a table of cell sizes shows just how small.