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 coal producer Walter Energy (NYSE: WLT) were burning hot today, gaining as much as 30% in intraday trading on the strength of buyout rumors.

So what: The Times of London unleashed rumors today that Anglo American may be preparing to bid as much as $120 per share for Walter. The Times also suggested that BHP Billiton (NYSE: BHP) may also be mulling an offer for the company. After a very disappointing stretch for the stock, investors would get a significant boost from the rumored offer.

Now what: Buyout rumors can be very exciting for shareholders. Walter's stock closed at $75 yesterday, so a $120-per-share buyout offer would represent a massive 60% premium. However, buyout rumors sometimes prove to be nothing more than that -- rumors. For that reason, investors chasing the stock's big gains today may want to be careful, because if an actual offer never materializes, traders with a "what have you done for me lately" mentality will eventually lose patience and the stock will tumble all over again.

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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.