Non-Newtonian fluids are fluids whose visosity changes according to the force which is applied to them. Stick your finger in slowly, and it behaves like a liquid, but bang it with a fist, and it behaves like it’s solid. Like in this film: Non-Newtonian fluid (Youtube link).
Or, more spectacularly, you can even walk on the liquid, like in this movie: Walking on a cornstarch-water-mixture (noisy Youtube link, in Spanish).
Of course, the best-known non-Newtonian fluid is Silly Putty, at least to Americans. By the way, I’ve never seen it for sale in Germany, but John VanDyk was kind enough to send me some years ago. Hi, John!
Non-Newtonian fluids have some interesting properties (the following are all Youtube links, except where noted):
Kaye Effect: Pour a thin stream of shampoo into a container, and a stream of shampoo might jump back up. This effect became famous when it was explained by a group of Dutch scientists last year, who produced a stunning video of the Kaye effect. See also Leaping Shampoo and the Stable Kaye Effect (PDF).
Fano Flow: the fluid can be syphoned tubeless – it seems to move upwards unsupported.
Weissenberg Effect: The fluid climbs up a rotating rod. You know this one from beating egg-whites with an electric mixer.
Barus Effect (aka Merrington Effect, Die Swell, and Extrudate Swell): After passing a small orifice the diameter of the liquid stream increases.
Liquid Rope Coil Effect: When the poured liquid reaches the surface, it seems to coil like a rope. You can demonstrate this one fairly easily with honey.