Plasma.

This is my sons plasma lamp. It used to be mine but rather than spend time in the shed collecting dust, it now spends its time each evening arching away while my son watches as he drifts off to sleep. I love these sorts of things that encourage the imagination. It's not just a straight forward lamp, rather, something that makes a young child stare and imagine how this thing works, what makes it do what it does. Even after understanding how it works (similar to the lava lamp), it still leaves you staring and appreciating its effect.