Oxygen is a gas in the air. It is a colourless gas without a smell or taste.
Oxygen readily combines with many substances, causing them to give out energy. For example, when oxygen and hot wood combine they give out heat. We call this burning. When we take oxygen into our bodies it combines with food to release energy.
When firefighters try to put out flames they try to remove oxygen from around the hot material. They may cover the burning material in foam or sand or use water to keep the oxygen away and to cool it down.
Oxygen can also make some materials change even when they are not hot. If iron or steel is damp, oxygen in the air will make the metal go rusty. These metals need to be coated with paint or a protective metal or plastic to keep water and oxygen away from them.
