Living cells carry out thousands of chemical reactions. At body temperature most of these would be far too slow to keep an organism alive, so cells make enzymes. An enzyme is a biological catalyst: it speeds up a reaction in a living thing and is not used up.
Enzymes are proteins, and each enzyme speeds up one particular reaction. Digestive enzymes in the gut break down large food molecules, such as starch, into small sugars that can be absorbed. Yeast cells contain enzymes that turn sugar into ethanol and carbon dioxide. This process is called fermentation and is used to make bread and alcoholic drinks.
Enzymes work best in mild conditions close to body temperature. If an enzyme gets too hot it loses its shape and stops working. It is then said to be denatured.