When I was a kid, I hated putting toothpaste on my toothbrush. So I invented a toothbrush with a hollow handle and hollow bristles so that you only had to fill the toothbrush up with toothpaste about once a week. I made some drawings of it, but never made it. This was in 4th or 5th grade.
Well turns out someone invented one and called it the "travel toothbrush". Not sure if that person is rich now, but they got my invention.