Solids, fluids and gases. We are used to them in our day to day life. We breath the air, drink the water and eat the apple. We know that below 0 degrees Celsius water is frozen into ice. At room temperature water is a fluid and above 100 degrees Celsius it vaporizes. We live with the idea that matter is either a gas or a fluid or a solid.
Yet there are many materials that cannot be so easily classified. These materials can behave as a fluid but also as an elastic solid. That gives them properties that are really stunning as shown in this video on Youtube: https://youtu.be/nX6GxoiCneY.
We call these materials viscoelastic materials because, at the same time, they have both fluid (viscous) properties and elastic properties.
What is causing these viscoelastic properties? The answer is astonishing simple: any material that consists of long flexible fibre like particles is in nature viscoelastic. Because of their shape the particles can temporarily connect to each other which causes the elastic properties. On the other hand, due to their flexibility, they will easily slide along each other which causes the fluid properties.
Typical examples of viscoelastic materials are spaghetti, shag (tobacco), a pile of worms moving through each other and (of course) polymers. Polymers are always viscoelastic because they consist out of long molecules which can be entangled with their neighbours.