Liquid Glass

Taken from The Right Attitude to Rain by Alexander McCall Smith.

This not be my review, but this made me laugh.

The glass in the fanlight, Jamie had said, was the origianl pane put in
when the tenement was built in 1850. "You can tell old glass," he
said. "It is thicker at the bottom than at the top. It's liquid, you
see. It very slowly sags downwards." Like people, thought
Isabel.