Hydrogen gas is a colourless, odourless, and tasteless gas. It is the most abundant element in the universe, making up about 75% of all matter. Hydrogen gas is highly flammable and is often used as a fuel in rockets and other forms of transport. It is also used to produce electricity and is a key component in the production of many chemicals. One example of a hydrogen compound is water (H2O), which is made up of two hydrogen atoms and one oxygen atom. Other examples of hydrogen compounds include hydrogen peroxide (H2O2) and ammonia (NH3).