How Lili recharged her mobile in Bulgaria

Every object possesses energy. Influenced by different forces every kind of energy can be transformed into another kind of energy. For example, the energy of movement (kinesthetic energy) can be turned into electricity. My friends from Bulgaria used this knowledge to recharge my mobile.  

They built a mill whose blades turn when hit by marbles falling because of the gravity. This way the energy of the balls turns into energy of movement and it turns into electricity to recharge my mobile.