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 PetSmart, (NASDAQ:PETM) were looking clever today, climbing as much as 15%, and finishing up 12.5% after JANA partners disclosed a stake in the pet-supply retailer. 

So what: The activist hedge fund revealed in an SEC filing that it had accumulated a 9.9% stake in the company, making it PetSmart's largest shareholder, and said it aimed to push for a sale. In a statement, PetSmart management said it "welcomes open communications with its shareholders and values constructive input toward the goal of enhancing shareholder value." Jana Partners said it expected to hold a meeting with PetSmart management and its board about a sale. 

Now what: The prospects of a sale generally help lift a stock, especially a troubled one like PetSmart. The retailer posted negative comps in its most recent quarterly report, at -0.6%, its first drop in seven years, and it lowered its sales outlook for the year in the same report. Jana has a history of successfully driving sales of similar stocks, as it was behind the recent sale of Safeway to Cerebrus Capital Management. In the time between Jana's investment and the sale, Safeway's shares gained more than 50%. It's too early to say if Jana will be able to lead PetSmart to being acquired this time around, but at the very least, its proposition is likely to put a floor on the stock.

Jeremy Bowman has no position in any stocks mentioned. The Motley Fool recommends PetSmart. Try any of our Foolish newsletter services free for 30 days. 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 has a disclosure policy.