Children are the Future

For one of my classes I need to think of an idea that promotes child development. So I came up with a new toy. It's a round object that can come in different sizes and colours. It's hollow, filled with air, so it's very light. It can be thrown around, rolled, sat on, etc - depending on the size. The best part of this new toy is that the number of people playing with it at one time is unlimited - one child, two, three, four, etc. The possibilities are practically endless! I just don't know what to call it...