Sodium Polyacrylate is the polymer used in diapers to absorb large quantities of liquids. You can get it from Educational Innovations, among other sources. It's great for "disappearing" water magic tricks. You have a small amount of the powder in a cup. Pour in some water and a minute or so later, turn the cup upside down and nothing comes out. (For your performance, you'll want to use an opaque cup, but I wanted to let you "in" on the action).
Ghost Crystals (also available from Educational Innovations) are lots of fun. This are much larger crystals than the sodium polyacrylate. After you've enlarged your crystals, very carefully tie a thread around a single crystal. Fill a water bottle with water and submerge the crystal, on its leash, in the water. Leave the thread hanging over the side of the water bottle and screw on the cap. Tell your students you've brought your ghost pet to visit for Halloween day. They'll look and look, but all they'll see is a leash suspended in the middle of the water!
It's also fun to trick your student with some "chunky" water. A clear container filled with the enlarged crystals and a small amount of water will look just like a glass of water. Fill a pitcher with the polymers and offer to pour a student a glass of water - instead of a stream of water, chunks will come out of the pitcher.
Other places to find water-absorbing polymer crystals:
Steve Spangler's Water Jelly Crystals look to be of a similar size to the ghost crystals, if you prefer to shop there.
I've seen similar products (I'm not sure of their exact chemical make-up, but they function in the same way) at:
Toy stores (random locations within the store)
Garden centers (as an additive to retain moisture in soil)
Craft Stores (usually in the floral area)
Science supply catalogs
Lots of fun for Halloween or April Fool's Day, but also useful when studying polymer chemistry!