Money you can see vs money you can't

Long ago, people traded goats for grain and shells for spears. That was hard, because not everyone wanted a goat! So people invented money: something everyone agrees is valuable, so trading becomes easy.

Today most money isn't even paper or coins. When your parents pay for groceries with a card, no cash moves. A number just changes in a computer somewhere. That's called digital money, and you've been using it your whole life without realizing it.