How this experiment works! The force of the water from the tap (source of water) spins the waterwheel, which is connected to a gear which spins another gear which spins another gear etc... in total there are 6 gears which spin together. The last gear is connected to a generator (which generates power) and the generator is connected to a light bulb. The power from the water pushing the water wheel lights the light bulb. Now, this all might seem quite complicated but actually, it's quite simple. Here's a simpler way of explaining it: the water turns the water wheel, the water wheel turns the gears, the gears are connected to a generator, the generator is connected to a light bulb.