How is it possible that non-Newtonian liquids harden when you hit them and turn back to liquid almost immediately. How did that happen?

Answer

A Newtonian fluid is a fluid whose viscosity is independent of the speed of the flow. Many liquids (including polymers, but also many nutrients) do not meet this requirement.

Polymers are large elongated (snake-shaped) molecules that are weakly bonded laterally (compare it to a plate of spaghetti). If you give the molecules time to move in relation to each other, the substance reacts in a soft, liquid-like way. But if you want to deform the material quickly, the molecules simply do not have time to move in relation to each other and the substance reacts ‘hardly’.

A fun toy to illustrate this is “Putty” of which you can find amazing videos on the net.
http://www.puttyworld.com/watchitshatter.html

There are other examples of non-Newtonian fluids:
Some materials are clearly ‘solid’, but if the shear force you exert on them exceeds a certain critical value, the substance suddenly becomes ‘liquid’. A good example is butter spread, which is obviously solid as long as you leave it alone, but starts to flow when you apply pressure to it. Snow and mud also have similar properties and can start to flow if they lie on an inclined surface with a sufficiently large slope.