Let it whoosh! The 2 kg sphere provides thrust for the rocket – thanks to muscle power and gravity. When it’s raised to a height of 3 metres, the sphere stores 0.00002 kilowatt-hours (kWh) of energy, which it releases on impact. Ready for take off?

Of course, a real rocket could not be launched like this. But the Earth’s gravity is an inexhaustible source of energy. This is harnessed, for example, in gravity energy storage systems:

Here, surplus energy from renewable sources is used to lift blocks weighing several tonnes. When the weights are released, the gravitational energy is converted into electrical energy – the same principle as in pumped-storage hydropower.