ROXSOL and RUSTY got the answer

The idea for this handy little invention came to the inventor during choir practice at church. What is it?

A: Post it notes.

3M scientist Dr Spencer accidentally made a glue that didn’t quite stick permanently and another scientist, Art Fry thought of how to use it while at choir practice at church.

3M scientist Art Fry wanted bookmarks to stick to the hymn book without damaging the paper.

They tested it around 3M and it was a hit.

This weekend's question is

This invention, dating back tens of thousands of years, was crucial for human development and progress and allowed them to plan and organize in ways never known before. What was it?