Magnetic Fluid or ferrofluid is amazing stuff. It's just runny black oil-like liquid until you put a magnet close to it. With a magnet nearby it forms itself into a solid-like substance. When the magnet is removed it collapses back into liquid form.
But wait... that's not all. Top Men are not sure who thought of this (probably the same guy who thought of coffee*) but when you put a drop of this ferrofluid between two glass plates and subject it to powerful rotating magnetic fields, really bizzare patterns appear. Here are some QuickTime movies. The second one starts off slow - but stick with it 'till the stuff at the end. Very weird.
UPDATE: Tom B. sends a link to a beautiful photo of ferrofluid at National Geographic. (Thanks Tom)
(*Thought process of the guy who invented coffee: "I have an idea. Let's pick these red cherry things, then dry them in the sun for a week or two, then throw away the fruit and keep the seed, then bake the seed for ten minutes, then grind up the burnt seed, then pour hot water through it, and then drink the brown liquid.")