A long wait, on this Moment of Science.   You might have heard the old adage that glass is a liquid, and if you go over to Europe and look at the stained glass windows in the cathedrals, you'll find them thicker on the bottom than the top because they are gradually spilling out. Is this true?

Before we answer that one, let's take a second to think about what a cool image that is. Maybe if you lived for centuries instead of years, you could sit around and watch glass pour the way we watch molasses ooze out of a jar.

Well, sometimes little factoids like this get passed around that may or may not be accurate. So, to set the record straight, glass is indeed a very slow-moving liquid -- yep! -- given the standard definition of a liquid, which is something that takes the shape of its container. If you could wait a really, really long time, you might be able to watch a seemingly-solid piece of glass do this.

But it would take a lot more than a few centuries. In fact, a recent study by Edgar Zanotto in the American Journal of Physics showed that the amount of time you'd have to wait to see glass change its shape with the naked eye would be longer than the age of the universe! Those cathedral windows are probably thicker in some places just because of the crude methods used to form them.

So, yes, technically glass is a liquid, but no, you can't ever see it move. Unless you have a few billion years to kill.