Iron egg

An iron egg is not a real egg made of iron, but a special object made to look like an egg that is actually made of iron. It was used in ancient China as a way to test if coins were real or fake.

Here's how it worked: People would take a suspected fake coin and rub it against the iron egg. If the coin left a mark on the egg, it was real. If not, it was fake. This was because real coins were made of softer metals that would leave a trace on the iron egg, while fake coins were typically made from harder metals that wouldn't.

So, the iron egg was used as a tool to help people in ancient China determine if their money was real or fake. It's a pretty clever invention!