Hydrogen rocket
First, we split water, using an electric current, into hydrogen gas and oxygen gas. This process is called electrolysis (2H2O becomes 2H2 + O2). The gases collect in the space between the water and the rocket.
Then, we ignite these gases.
This releases so much energy that the rocket shoots upwards.
The hydrogen gas and the oxygen gas combine to form water again (2H2 + O2 becomes 2H2O).
The same principle is used in cars that run on hydrogen gas.