$20 interest from your bank in a year probably sounds a lot to your 10 year old, but he wouldn't understand that with current banking practices you probably need $2000 to earn that
...actually about $370-$450 in capital should yield $20pa. I wouldn't lecture in economic theory if your cash is earning you 1% when 4.5% - 5.4% cash management rates are readily available to the average punter.
Rudderless1 says it all really quite succinctly.