GLASS IS ACTUALLY DEFINED AS A LIQUID OF A VERY GREAT VISCOSITY
GLASS DEFINITION glass  (noun) [Middle English glas, from Old English glaes; akin to Old English geolu yellow -- more at YELLOW] First appeared before 12th Century
1 : any of various amorphous materials formed from a melt by cooling to rigidity without crystallization: as
a : a usu. transparent or translucent material consisting esp. of a mixture of silicates
b : a material (as obsidian) produced by fast cooling of magma
Many people believe that glass "flows" like a liquid, and the proof most often cited is that stained glass windows in ancient cathedrals are thicker at the bottom than at the top. The idea that glass is a highly viscous liquid at room temperature has even made its way into some textbooks.
Someone finally decided to test the idea, and it turns out to be wrong. Edgar Dutra Zanotto, a professor of materials engineering at the Federal University of Sao Carlos in Brazil, looked up the chemical composition of some 350 pieces of glass from 12th century cathedrals, calculated their viscosity, and then determined their flow rates by extrapolating the viscosity curves of hot glass to lower temperatures.
According to Zanotto's calculations, you would have to heat a typical 12th-century piece of glass to approximately 414 degrees C to observe any significant movement in the course of 800 years. Without high heat, you would have to wait about a hundred million trillion trillion years to observe any flow, far longer than the age of the universe.
FOURTH STATE OF MATTER
What makes glass a unique material is that it's always a liquid; glass is known as "the fourth state of matter" because it has no solid or gaseous state. Even though windows and wine goblets seem to be solid, the glass they're made of is actually a super-cooled liquid whose molecules are moving very, very slowly. As the glass heats up in the kiln, its liquid nature becomes visible. At about 1450oF, you can see the edges of the cut glass starting to soften and melt. At 1550oF, most glass is beginning to actually flow and behaves like syrup (extremely hot syrup!), and as its temperature continues to rise, you can actually see it moving in the kiln.