While on the course, I met a group whose exhibit will be about polymers. They brought along a few for us to play with, including a perennial favourite, sodium polyacrylate. This is the stuff used in diapers, as it can absorb 200-300 times its own weight in water. If you make a sphere of it and soak it overnight, it takes on all the optical properties of water. Therefore, if you drop it in a glass of water, it seems to disappear. Like so (YouTube video, 24 seconds):
The second video shows a section scan through my brain (YouTube video, 15 seconds). Ooer.