Do glass panes become thicker at the bottom over time? Is it true that a glass window, that has been placed in a wall for about 10 years or more, is thicker on the bottom than on the top?
I can vaguely remember my physics teacher saying that this was true.
So if this is true, how is this possible?
 A: 
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries. The assumption being that the glass was once uniform, but has flowed to its new shape, which is a property of liquid. However, this assumption is incorrect; once solidified, glass does not flow anymore. The reason for the observation is that in the past, when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk became a different thickness as the glass spun. When installed in a window frame, the glass would be placed with the thicker side down both for the sake of stability and to prevent water accumulating in the lead cames at the bottom of the window. Occasionally such glass has been found thinner side down or thicker on either side of the window's edge, the result of carelessness during installation.

Wikipedia
A: Cause glass is an amorphous substance and with time they can flow like liquids. Because of this they're also called pseudo solids or super cooled liquid.
