Hydrogen and oxygen, when combined in the right proportions and with the right conditions (like a spark or heat), produce water (H₂O).

This is a very common and important chemical reaction, represented by the following equation:

2H₂ + O₂ → 2H₂O

This means that two molecules of hydrogen (H₂) react with one molecule of oxygen (O₂) to produce two molecules of water (H₂O).