Updated from 1:11 p.m. ESTIf you were watching the biggest-percentage gainers on Thursday, you would have noticed that Sears Holdings ( SHLD), which trades on the Nasdaq, was up by almost 9.5 points, an increase of more than 23% in just one day. This was likely due to a short squeeze. The stock has 16% of its shares shorted, and when the company's fourth-quarter earnings beat analysts' estimates, short-sellers scrambled to cover their positions, driving the stock much higher. There are also plenty of heavily shorted stocks on the NYSE (including USEC ( USU)). Short-squeeze opportunities are measured by the short ratio, which is the number of days it would take the short-sellers to cover their position based on recent average daily volume. To read more, visit Stockpickr.com. In Monday trading, USEC closed at $5.30 per share (up 18% year-to-date).