The Photosynthesis Robot is designed to be driven by light via plants and how they work in conjunction with photosynthesis.
The idea is that light from the sun comes to the earth and the plant absorbs it. Then, through phototropism - the movement of plants towards the direction of the sun - the cart it’s on moves very slowly with the plant on it moving torward the sun.
A cool experiment indeed, but don’t ditch that hybrid Prius just yet.