The short answer is: By investing their earnings back into their original investment.
The long answer is let your hard earned money work for you. This may take several years to accomplish but by investing your earnings back into their original investment you will realize gains much larger then if you did not. This process called compounding which basically takes any earnings from your assets (capital gains and/or interest) and reinvests it to get more earnings over time. This type of growth generates earnings from its initial value plus it accumulates earnings over time on top of any added earnings.