Reality Check - Margins 

Many investors in the real world use "margins".  This is essentially a loan from your broker allowing you to buy more stock than you could otherwise afford.  For example, a 50% margin means you only need to provide 50% of the money necessary to purchase a given stock.

This allows investors to increase their positions and perhaps earn greater returns, however it also increases risk.  If an investor's portfolio falls and they fall below the minimum margin, they are subject to a "margin call", where the broker literally calls the investor and asks for additional money.