Way back when I was a kid in second grade one kid did not have money for milk. I gave him 25 cents to get some milk under the terms that 25 cents would be added to the amount owed every day. He did not pay me back the next day. The day after that I reminded him his total was .50. I kept reminding him every day until he paid me $2.50. Later on my parents asked me where I got all that money and I told them I got it for letting a kid borrow milk money. When they asked why I got that much instead of 25 cents all I said was "Interest."
My parents didn't like that response and made me give that kid back all the money minus 25 cents. If only they encouraged my ability to recognize investment opportunities at a young age I could have been a more successful man today.