This may be more of a scientific question than a historical one.
However...
Most solids have a rigid and crystalline structure, in which the molecules are arranged in regular, repeating patterns. The arrangement of molecules in glass is non-crystalline, non-regular, and non-repeating. At this molecular level, glass "looks" more like a liquid than a solid.
It has been observed that the glass in very old windows (centuries old) is thicker at the bottom of the pane than at the top. This was taken as evidence that the glass has slowly (very slowly) flowed from the top to the bottom of the pane. It turns out this is actually a side-effect of the the production process.
Bingo: glass is an extremely slow-flowing liquid. Obviously.
