Rosetta Stone.
It's basically this slab that held two different languages on it. Those being Egyptian and Greek. It had a bunch of different type of script on it, too.
A man studied it and a bunch of other stuff for forever before finally being able to use it to understand hieroglyphics. This was the case because he had studied languages like Greek and Coptic before, knowing them, which helped him use the Rosetta Stone to unlock the meaning behind Egyptian hieroglyphics.
