Actually, I believe that both answers are correct. I think the real question is, "What is a glass?"

Liquids, solids, and gases are all made up of atoms. Now you may have learned that in a liquid, the atoms are all jumbled and tumbled together.

Most solids, on the other hand, are crystalline, which means that the atoms are neatly arranged. Think of billiard balls all stacked tightly together, or maybe the ball and stick models you may have seen in class.

Now, a glass is somewhere in between. The best way to think about a glass is to imagine the liquid and imagine that you can actually see all of the atoms moving around. Now take a picture. When you look at the picture, the atoms are frozen in time and place. This is a glass.

Think of what happens when you freeze a glass of water. At first, the atoms in the water are moving around because the water is warm and they have a lot of energy. As the water cools, the atoms move more slowly. As they lose energy, the tend to arrange themselves in the best possible manner and form ice crystals. However, this takes time. If you cooled the water so quickly that the atoms couldn't move to the best locations, you would have a glass.

This is what it means to say that a glass is a liquid with a very high viscosity. The atoms would like to move to a position that has a lower energy, but they're stuck or frozen in place instead.

I once read a definition that said if you were to kick a piece of material with you barefoot, and it hurts, then it's solid. That might be a silly definition, but it's pretty good. The windows in your school are glasses. They're solid, too.

Hope this helped.

You are asking a question which is really at the edge of our current knowledge of physics. In fact, there is an entire sub-field of physics designed to answer exactly this question. So, the short answer to your question is that, according to a glass expert down the hall from me, nobody knows.

In many solids, the atoms of the material arrange in a regular pattern called a crystal. In these materials, there is a sharp transition between the liquid and the solid, and quite a bit is known about this transition. In a glass, something much weirder happens. As you cool the liquid down, the molecules slow down and start hitting each other more and more. As this happens, they begin to get stuck on each other, until eventually, they are so stuck that they are unable to flow. This is called "jamming". What seems to happen is that the time it takes for a glass to flow gets higher and higher, until eventually it takes thousands, or hundreds of thousands, or billions of years. So, does glass flow? Yes. Is it a liquid or a solid? It depends on how long you look at it. I would probably tell you that it was neither - it's just a glass.

Just to show you how weird glasses can be and to give you an example of what I mean, there are materials (such as pitch tar) that can be poured into a funnel. Every 80 years or so, one drop will fall from the bottom of a funnel. If you smash it with a hammer, however, it will shatter.

So over an 80 year period, it is a liquid. Over the time it takes to smash it with a hammer, it is a solid.

The last thing I want to say is that I've heard that stained glass windows in old churches look as if they have flowed. Apparently, there is still a big argument about whether that glass has flowed over a few hundred years or if the glass looks that way because of how it was made.

This is a good question. The difference between a crystal and a liquid is that a crystal has a long range periodic structure that goes on forever . That is, once the unit cell is determined, one simply repeats this UNIT CELL throughout space.

A liquid on the other hand is a DISORDERED material ... It has only a very short range structure ; beyond a few atom diameters liquids are significantly disordered---they possess no long range structure.

Now, a GLASS is frozen liquid... so while a glass has the thermal properties (like the specific heat) of a crystal , it shares the disorganization at long range with a liquid.

So a glass is a solid ; but it can also be described as a frozen liquid (of very high viscosity).