Polyacrylamide crystals can absorb over forty times their weight in water and also have interesting optical properties. Students enjoy seeing and learning about their surprising behavior.
If you are studying hydroponics, students can make artificial soil with polyacrylamide crystals and water. Once the crystals have fully expanded, put them in a petri dish and your students can study them with a hand lens. These crystals also have the interesting property that the speed of light through the crystals is the same as the speed of light through water, so they become invisible in water. Once you have shown your students this, dip your hand in to pick up the crystals and show that they are still in solid form.