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 Family Dollar (NYSE:FDO) and Dollar General Corp. (NYSE:DG) were moving higher today, jumping as much as 16% and 14%, respectively, and closing 13% and 7% higher after activist investor Carl Icahn disclosed a 9.4% stake in Family Dollar.

So what: The renowned billionaire investor revealed his holding in the discount chain after hours Friday, and Family Dollar promptly responded by adopting a "poison pill" shareholder rights plan, which prevents any shareholder from amassing more than a 10% stake in the company. In his filing, Icahn called Family Dollar "undervalued," and said he may seek seats on the board to enact changes. Family Dollar recently lowered its outlook for the year and has changed its strategy to and everyday low prices model from a high-low model, which could make it ripe for activist influence.

Now what: Dollar General shares rose as investors speculated that Icahn's investment could lead to a merger between the two dollar-store chains. It's too early to say what changes could come to Family Dollar or Dollar General as a result of Icahn's move, but the famous investor has made some smart bets recently on companies such as NetflixHerbalife, and Apple and has been key in inducing the iPhone-maker to return more capital to shareholders. Given his investing resume, Family Dollar shares simply just because he took a stake in the company and sees it as an undervalued opportunity.

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Jeremy Bowman owns shares of Apple. The Motley Fool recommends and owns shares of Apple and Netflix and has options on Herbalife. Try any of our Foolish newsletter services free for 30 days. We Fools don't 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.