To quote The United States Securities and Exchange Commision

 What is a Ponzi scheme?

A Ponzi scheme is an investment fraud that involves the payment of purported returns to existing investors from funds contributed by new investors.

Ponzi scheme organizers often solicit new investors by promising to invest funds in opportunities claimed to generate high returns with little or no risk.

In many Ponzi schemes, the fraudsters focus on attracting new money to make promised payments to earlier-stage investors and to use for personal expenses, instead of engaging in any legitimate investment activity.

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Why do Ponzi schemes collapse?

With little or no legitimate earnings, the schemes require a consistent flow of money from new investors to continue.

Ponzi schemes tend to collapse when it becomes difficult to recruit new investors or when a large number of investors ask to cash out.

And there you have it.

Social Security as explained by the SEC.