In the book Rich Dad Poor Dad, Robert Kawasaki teaches that in order to become richer you simply have to acquire more assets than liabilities. Having assets that generate reliable cash-flow allows you to reinvest and take advantage of compound interest.
Diversification is also important. It allows for failures while still being able to turn a total net profit. Below is a list of assets that you can invest in in order to acquire a reliable source of cash-flow.
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Energy and Commodities