Oxygen is a colorless, odorless gas that makes up about one-fifth of the air around you. Every breath you take pulls oxygen into your lungs, where it enters your bloodstream and travels to every cell in your body. Your cells use oxygen to convert food into energy, which is why you need to breathe constantly to stay alive.

Oxygen does more than keep living things alive. It's also essential for fire: flames need oxygen to burn, which is why smothering a candle cuts off its air supply and puts it out. When things rust, that's oxygen slowly combining with metal. When wood burns in a campfire, that's oxygen combining with the wood rapidly, releasing heat and light.

Scientists represent oxygen with the symbol O because oxygen is an element. Oxygen gas is written as O₂ because oxygen atoms usually travel in pairs.

Your body is incredibly sensitive to oxygen levels. If you hold your breath for even a minute, you feel an urgent need to breathe. Climbers on tall mountains need oxygen tanks because there's less oxygen in the thin air at high altitudes.