My dad wanted to show me the power of interest when I was little (under 10 years old), so he started "The Bank of Dad" and gave me 12% annually on any savings I had him hold. He trusted me to track it honestly on my own in an excel spreadsheet, checked my work the first few months to make sure I was doing it correctly, but then went completely hands-off and didn't do much except accept the occasional cash deposit.
Somewhere in my mid teens, he noticed I was giving him more deposits since I'd started working part time... he asked to see my records and realized the interest on my little $2k account was $20+ a month and compounding heavily. Quickly shut TBOD down after that, but helped me research mutual funds and set me up with a Vanguard account. Talk about awesome parenting.
To be completely honest, though, I'm still a little upset that I'll never see that risk-free 12% return again. Way to ruin banks for me forever, Dad.