The Floating Sinker

The best toy for children 4 and up.

• Your child will experiment with gravity and buoyancy with this expanding cylinder, which, at the flip of a switch, changes volume without changing mass. As any good physicist knows, this will change the density, and thus buoyancy of the toy, and will determine whether it floats or sinks in various liquids.
• This toy is very durable and safe, as there are no small or weak parts. The motor may be burned out if there is excessive resistance. Manufacture of the toy would be between $20 and $30, depending on how well machined the pieces need to be.