A Ponzi scheme is an investment fraud where returns to early investors are paid from funds received from later investors rather than actual profits. As more investors are needed to pay promised returns, the scheme relies on a constant flow of new money to avoid collapsing. When new investor money dries up, as it inevitably will, the scheme falls apart leaving most investors losing their money. Investors can protect themselves by being wary of unrealistic returns, doing thorough research on any investment, and watching out for common red flags of Ponzi schemes like vague explanations of how profits are made.