What is the ancient coin (kopek?) that peeps used to carry around in their mouths?

I’m trying to confirm a vague remembrance from 5th grade social studies (orsomethinglikethat) of a fact about coins that people would carry in their mouths. I thought it was called a kopek, but all I can get in online research is the Russian coin or some derivative. It would be a round coin with a hole in it, and it would have belonged to some cradle-of-civilization-type society. Any ideas? Am I making this up?