The cub just learned the meaning of money.... he has a handful of pennies his trains have been carrying around as freight and he told me he wanted to use "a money" to buy a toy. So, okay, one lesson at a time? I let him carefully carry the penny to the store where I had to get some things anyway, and let him pick out a toy. Then he very carefully gave the penny to the cashier (and she winked at me and palmed it back into the bag) and got his toy.
Theennnn he discovered that when you buy a thing, you don't get your coin back.
After much deliberation he decided to keep his toy and let the cashier keep the money.