Jail cells do not have windows!

From the Fish Don't Notice Water file:

A standard feature of movie jail cells seems to be a window with bars and no glass. The hero can chat with friends standing outside or (more commonly) receive escape materials through the window. What gives? This is some sort of cultural QWERTY phenomenum, a cliche so engrained it doesn't get changed. It's been decades since a jail cell had glassless windows, or even windows at all.