Make a flashlight whose energy source is light. Despite not having a battery, the flashlight will work fine at night.
No, this isn’t a trick project. It actually works. Since the energy source for this flashlight is light and it works just fine at night, it must have a way of storing energy–and it does. However, like commercial solar powered flashlights, which store chemical energy in batteries which then convert this chemical energy to electrical energy, this flashlight stores electrical energy directly in two one-farad super capacitors. Today we use two 1-farad capacitors to store electrical energy to light an LED; tomorrow we will be using a bank of 10K farad capacitors to store electrical energy to power an automobile!