What is the white stuff shop keepers used to use to write on their windows?

Shopkeepers traditionally used a substance called "window chalk" or "whitewash," which is a mixture of water, chalk, and sometimes lime, to write on their Windows. This material allowed them to create temporary signs, advertisements, or promotional messages. It was easily removable with water, making it ideal for changing messages frequently. Today, similar products like liquid chalk markers are commonly used for this purpose.