Answer: Cellophane Tape

Frosting glass by etching the surface with an acidic compound or sandblasting it is a common way to permanently increase the privacy and security offered by the glass. It would appear to be a completely irreversible process short of painstakingly polishing the glass until the etched layer had been completely removed, but there’s a neat little trick that allows you to look through security glass that is only etched on one side: you can press a piece of cellophane tape over it.

This bit of office-supply wizardry works because the cellophane tape temporarily fills in the irregularities of the etched surface (you could also approximate the same effect, albeit in a much messier fashion, by smearing oil over the glass). The result isn’t as perfectly clear as a regular sheet of glass would be, but you can still easily look through it and identify what is on the other side.