Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Apollo Education Group Inc (APOL) were getting an "F" from investors today, falling as much as 14% and closing down 9% on a rumor that hedge fund manager Bill Ackman has taken a large short position in the stock.

So what: The parent of the University of Phoenix started falling shortly after 11 a.m. as word spread that Ackman's Pershing Square fund was soon to announce a short position in the for-profit educator. Trading volume in Apollo shares was nearly six times its normal level, though it's unclear when the shares would've been shorted. The education sector was falling across the board today as peer ITT Education Group withdrew its guidance as it awaits a decision from the SEC that could affect some of its previous results.

Now what: There was no confirmation of the Ackman short, so for now the news remains just a rumor. While Ackman is one of the more closely followed hedge fund managers, he has a made a number of big-time blunders recently, including backing J.C. Penney and former CEO Ron Johnson and shorting Herbalife, whose shares are up close to 50% since he announced his position. Apollo, like much of the education industry, is struggling with declining enrollment, which has sent revenue and profits plummeting. Whether or not Ackman is short, expect the company's fundamentals to continue driving the stock, not the opinion of one investor.