About 5,000 years ago, 30 goats changed hands between Sumerians. To record the transaction, a receipt was carved onto a clay tag, about the size of a Post-it. Simple geometric signs represented the livestock and purveyor. The indents of circles and semicircles denoted the quantity exchanged.

Imagine how surprised these people would be to learn their receipt is now held in a museum.

That’s because the tag is one of the earliest texts from the oldest known writing system, Mesopotamian cu