Coal tar pitch seems hard, but it is not. In fact, it is a very viscous liquid, i.e. he is liquid. Viscosity is a measure of resistance to spreading. Olive oil is about 100 times more viscous than water, and honey is 100 times more viscous than oil. The viscosity of pitch is 230 billion times greater than the viscosity of water. At the University of Cleveland, the world's longest running experiment on pitch is underway. In 1927, pitch was placed in a funnel. For 90 years, only 9 drops have fallen from it. No one was present when the drop fell. In 1988, the curator of the experiment, John Mainston, came close to seeing a drop fall. He left the room to pour himself some tea and missed the treasured moment. You can watch this experiment online, but since the last drop fell in 2014, it is unlikely that you will be able to see the coveted fall in the coming years.

Another substance that is a viscous liquid is glass. Glass is unusual in that it is an amorphous body. Silicon dioxide molecules do not form an ordered structure. Glass cools so quickly that when it goes from liquid to "solid" state, the molecules don't have time to line up in an ordered crystalline structure. What makes glass visually hard is atoms or molecules that are chemically bonded so tightly to each other that they cannot slip next to each other.

However, the lack of an ordered crystal structure makes glass still liquid, even when it is in a visually solid state. It is precisely because glass is actually liquid that in window frames in old houses, where windows have stood for several decades, it is clearly noticeable that the glass is thinner at the top than at the bottom. This is due to the fact that some of the glass has already been glass from top to bottom over the years. Therefore, in such houses, the windows rattle in the frames, because at the top they are already thinner than the gap prepared for them. Sometimes this effect is so noticeable that even a gap is formed on top.

Move on. What do we know about the interior of the Earth? Under the earth's crust is the mantle, which is responsible for the movement of tectonic plates and earthquakes. Is it solid or liquid? We will never be able to see the mantle directly, but we can observe lava, which is hot rock. You can imagine that the mantle is very similar to her. The mantle has to be fluid because it needs to flow, right? Actually, no, because the mantle is a solid body. Earthquake propulsion waves can propagate through the mantle, but these waves cannot travel through liquids, which is proof of its hardness. How does a solid stone flow? The answer lies in the imperfection of crystals, which may be missing a few atoms. The viscosity of the mantle is similar to the viscosity of glass, only several orders of magnitude higher. The mantle becomes like a liquid, but only in geological time periods. Peck is a liquid that can flow so slowly that it appears to be a solid. And the earth's mantle is a solid that behaves like a liquid if you wait long enough.

Hardness and plasticity have no absolute value, but only a relative one. And all bodies are actually liquid and solid at the same time. When there is a huge mass and strength in the case, the difference loses its meaning. The rigid definitions we create for ourselves lead to misconceptions and viscous rumors.