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 health-care service provider Tenet Healthcare (NYSE: THC) are doing a fine impression of a dying swan today, falling as much as 20.2% on more than 10 times the average trading volume.

So what: Tenet jumped drastically in January when rival Community Health Systems (NYSE: CYH) offered to buy the company. Today's freefall comes as Tenet rejects the offer in rather violent fashion, filing lawsuits against Community and generally kicking like a cornered mule.

Now what: Community's shareholders are suffering even more than Tenet's today, as the proposed merger of equals would have created enormous synergies and built an instant mastodon. Market leader SunLink Health Systems (AMEX: SSY) is the logical winner if this deal falls through, and that stock is naturally soaring today. I think a merger would benefit both Community and Tenet, but some highly qualified members of Tenet's board disagree. If this is a stalling tactic, designed to conjure up a better offer, it's a highly elaborate and risky one.

Interested in more info on Tenet Healthcare? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.