Melting Styrofoam with Acetone

What did that smiley face ever do to deserve that kind of treatment?

From about.com

Styrofoam is made up of polystyrene foam. When the polystyrene dissolves in the acetone, the air in the foam is released, causing it to look like you're dissolving this massive quantity of material into a small volume of liquid. You can see a less-dramatic version of the same effect by dissolving other polystyrene items in acetone.

...and a similar explanation but with nicer molecular structures drawn out.

So, if you missed it, here's the deal. Polystyrene foam is a solid with a while bunch of air surrounded by 'bubbles' of polystyrene. Think of a pile of soap bubbles.

The acetone breaks each of those bubbles as the polystyrene dissolves into the liquid. Effectively, the acetone 'pops' those bubbles. The gas inside - mostly air - goes away (you can see bubbles at the surface of the acetone in the above video. The polystyrene is still there - just like the bubble solution is there even after the bubbles pop. It's just the air inside the polystyrene bubbles (like soap bubbles when they pop) that is gone.

Still, though, it looks like magic.