3 opaque cups
Ahead of class time place some sodium polyacrylate in one of the cups.
During class, pour some water into the cup containing the sodium polyacrylate. Ask students to keep their eye on this cup and then rearrange the cups (like a shell game). As the instructor rearranges the cups students are easily able to pick out the spiked cup. However when the instructor inverts the spiked cup no water comes out.
In a variation, the three cups can contain a few drops of 0.1M NaOH, 1M HCl, and sodium polyacrylate. A cylinder is filled with water with some base indicator. As shown in the video the first cup turns the water pink, the second makes the water colorless, and the third makes the water "disappear".
Sodium polyacrylate is the absorbent used in disposable diapers. Osmotic pressure, the passage of pure water through a membrane permeable only to water, caused the superabsorbent polymer to absorb water. The difference in sodium concentration between the inside of the polymer and the solution in which it is immersed causes the water to rush in trying to equilibrate the sodium ion concentration inside and outside the polymer. The electrolyte concentration in the water being absorbed greatly affects the amount of water which can be absorbed per gram of polymer. In distilled water the polymer will absorb 800 times its own weight. In tap water, the polymer will absorb 300 times its own weight. In 5% sucrose the polymer will absorb 200 time its weight. In 0.9% sodium chloride it will absorb 15 times its weight.