It's a little more complicated than that. People like to say this as a fun factoid, but it's a little misleading and not strictly "true." It's just that glass has liquid-like properties but only over huge time spans. We define liquids and solids based on how the atoms of a given substance move relative to each other. Yes, over long periods of time, the atoms of glass can move around each other so that it can "flow," as a liquid does. But we're talking hundreds of years or more. Anything you do it that doesn't take that long, it will behave like a solid - it will be a solid. If you do something to it like putting pressure on just one part of a glass sheet and leaving it like that for many years, or putting directional pressure on it like with gravity, over centuries, it behaves like an extremely slow, viscous liquid. For all intents and purposes though, it really is a solid. It makes most sense to say that it's a solid that displays liquid properties over long timescales. Really, all of this is a matter of imprecise language and how "solid" and "liquid" don't make sense under certain (unusual for us) conditions, like at extreme temperatures or pressures, or in this case, over a very long time.

Edit: I was going to edit this comment to add some links to support this but Egon beat me to it with a better link.

This is not true