http://gadgetking.us/amz/B00MJXIEUY
What is a Ferrofluid? It is a colloidal liquid made from particles less than 10 nanometers in diameter. When this ferrofluid is put under a magnetic field the nanoparticles jump to attention and form cool spikes. Not exactly a kids toy, but fun nonetheless.
Check it out here for about $22 here.
