When I was a child, I remember wondering why there were no animals that could photosynthesize. Maybe that's a bit odd, but it's not that I was especially geeky; I just felt almost indignant that there weren't animals with green skin. It seemed to make so much sense.
Can't find food for dinner? No problem! Just make your own from sunlight, air and water.
Maybe I'd got the idea from "Swamp Thing" or "Black Orchid" or some other comic-book character that was half human, half plant.