Magnetic fluid is a lot of fun to play around with, especially if you are into science. You can interact with it using a strong magnet. This Dancing Magnetic Fluid Device takes it to the next level. It has a large viewing window and built-in LED backlighting.
As you listen to music around the device, the ferrofluid inside dances. You also have the option to manually interact with it with a handheld magnet.[where to get it]