The one thing that seemed to get through to my oldest was my snowball analogy.
Anyway, the analogy is that you start at the top of a big snowy hill with a little snowball. You drop the snowball and it starts rolling down the hill. As it rolls down the hill it picks up more snow, which makes the snowball go faster and faster. Eventually, that snowball has become so big and is going so fast that you can scrape off some of the snow as it rolls along and it will keep going, providing you with free snow forever.
I think the reason it worked is because he has an intuitive understanding about snowballs, gravity, and momentum and can picture it in his mind. Then he can transfer that intuition over to money, bank accounts, interest, etc.
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