A more thorough explanation:

A margin call is a demand made by a securities broker to a customer to put up money or stock as collateral when the broker finances a purchase of securities. This demand usually happens when the market prices of the securities are falling. It is also known as a maintenance call.

• John bought $10,000 worth of stocks on margin from his broker. The broker required him to put up $5,000 as collateral. However, the market prices of the stocks fell, and John received a margin call from his broker, demanding that he put up more money to cover the loss.
• When Sarah bought a house, she took out a mortgage from a bank. The bank required her to put up a down payment of 20% of the house's value. If the value of the house falls, the bank may require Sarah to put up more money as collateral, similar to a margin call.

These examples illustrate how a margin call works. When the value of the securities or property falls, the broker or bank may require the customer to put up more money as collateral to cover the loss. This is to ensure that the broker or bank is protected from any potential losses.