Here’s a little-known fun fact about money. The first credit card, or the idea of “charging” and paying later, started in the Middle Ages. In her new children’s book, Priceless Facts about Money, Mellody Hobson explains that knights in armor didn’t carry cash — too risky — and instead paid for things along their travels by using the stamp of their signet ring. The bill would then be sent to the king to pay.

Hobson, co-CEO and president of Ariel Investments, aims to demystify money and finances for children and to get them engaged in how to handle it. She sites her own childhood as inspiration, when her family struggled to pay bills. Her mother didn’t shy away from exposing Hobson from bills, bank statements and calculating tips. All of which built her confidence around eventually managing her own finances. And her message is clear to parents: don’t shy away from talking to children about money.