It's no secret far too many Americans haven't saved enough money for retirement. One reason why is because people don't start saving early enough to take advantage of compounding interest. 

That's why Shark Tank star Kevin O'Leary, chairman of O'Shares ETF Investments, started Beanstox an app that allows users to invest in fractions of shares of stocks. O'Leary said those who can't afford a share of Apple (AAPL - Get Report)  may be able to afford a fraction of a share.

Meanwhile, O'Leary is more bullish on the Russell 2000 index than the S&P 500. He said the small-cap companies in the Russell 2000 derive most of their revenue domestically and aren't as affected by currency fluctuations.

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