Peroxide is a chemical compound that contains two oxygen atoms bonded together. The most common type is hydrogen peroxide, a clear liquid that looks like water but fizzes and bubbles when it touches certain substances.

You've probably seen hydrogen peroxide in a brown bottle in the medicine cabinet. People use it to clean cuts and scrapes because it kills germs, though it stings a bit and creates white foam as it works. That foaming happens because enzymes in your blood and skin break the peroxide down into water and oxygen gas, creating thousands of tiny bubbles.

Peroxide is also used to bleach things lighter. Hair salons use it to turn dark hair blonde, which is why someone with bleached blonde hair is sometimes called a peroxide blonde. It can also whiten teeth, remove stains from clothes, and even clean surfaces around the house.

The chemical is stored in dark bottles because light breaks it down over time, turning it into plain water and oxygen. This is why old peroxide doesn't fizz much anymore when you pour it on a cut. While peroxide is useful in small amounts for cleaning wounds, it is quite strong and can damage healthy tissue if used too much, and doctors may recommend just soap and water for many small cuts.