The Glass Mystery Glass is a liquid when melted, but once it is cooled, it stays solid. However, some people disagree. They claim that glass is never fully solid, and that it “flows” very, very slowly. For example, old windows are thicker at the bottom than they are at the top. The argument is that, pulled by gravity, glass has been flowing to the bottom of the window for years.

Actually, older windows are not the same thickness all the way through because old glass-making techniques could not make flat sheets of glass like we have today. Part of a sheet was thicker than the rest. When putting windows into their frames, window installers put the thicker side of the sheet down. This made the window more stable and prevented the rain from leaking into the house.

More evidence points to the fact that solid glass does not “flow.” Antique telescope lenses have not changed shape since the day they were made.
Problem

What is the central idea of the article?

A

People have been making glass for thousands of years.

B

Glass does not “flow” once it is cooled and solid.

C

Window-making techniques have improved over the years.

D

Old windows look different from new windows.