Can glass deform over time?

I've seen glass that was about 100 or 150 years old, and it wasn't deformed at all. Glass becomes rather brittle after a while though, and I don't think it would last long enough to deform.

I don't think of glass as a liquid , it is only liquid when molten , and is mostly silica compounds .
I have seen glass that has deformed , usually by constant exposure to sun and heat .
The grade of glass has a lot to do with whether it will deform .

Wikipedia ( them again ) has an excellent set of articles on glass .

I've seen glass deform as well. Just from the sun.

Glass is what is known as an "amorphous solid." While it is indeed a solid, it's shape is not constant. Look at the windows in an old old house or building. They will be thicker at the bottom than at the top. That is because the crystal lattice that creates the structural stability for the glass, or makes it feel 'hard', is not completely solid. It moves.