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How did this day trader lose so much?
The day trader in the article was engaging in short selling.  Short selling is a technique used to profit when a stock goes down.  The investor borrows shares of a stock from someone else and sells them.  After the stock price goes down, the investor buys the shares back and returns them, pocketing the difference. As the day trader in the article found out, it is a dangerous practice, because there is no limit to the amount of money you can lose. The stock was trading at $2, and the day trader thought the stock was going to go down to $1.  He borrowed and sold 8,400 shares at $2.  He hoped to buy them back at $1 and earn $8,400 profit.  Instead, the stock went up a lot, and he was forced to buy back the shares at $18.50 per share, or about $155,400.  He had had $37,000 with E-Trade, which they took, and he is now over $100,000 in debt.