This takes some prep work, but it is downright stunning.  A small spoonful of innocent-looking white powder is stirred into a glass of water.  A few moments later, when the glass is turned up side down, the water is weirdly solid.  You can even shake the glass up side down!

This powder, “super-absorbent polymer” is used in disposable diapers to help prevent leaking.  Each little grain of powder swells into a gelatinous bead that traps water.  (See the last photo.)

Where can I find this stuff? The easiest way to get it is to tear apart an (unused!) diaper or two over a cookie tray.  Really rip apart the cottony insides of the diaper, releasing the white flecks of powder that are dispersed through it.  Tip the tray to shuffle the powder into one corner, and collect it.  Discard the rest of the diaper.

The Second Act. You’ve seen the first part: simply stir a spoonful of powder into a glass of water.  The water will be “solid” within a minute or two.  But there’s more!  Add some sidewalk salt (active ingredient: calcium chloride) to the glass and mix it in.  The salt effectively disables the polymer, and water comes back out.  Try it with table salt too (sodium chloride). What happens?

Safety Note. The super-absorbent polymer is safe – it’s in diapers, remember – but if you’re collecting lots if it, avoid breathing its dust.  Sidewalk salt, while not too serious, is less people-friendly.  Safety glasses and dishwashing gloves are recommended.  Wash the glass thoroughly after this demo.  If you do the part with the sidewalk salt, use a plastic cup that you can throw away.