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 Navistar International (NYSE: NAV) drove as much as 16% higher today before dropping to a more modest gain of 8% mid-day.

So what: The big news today is that billionaire investor Carl Icahn has purchased a 9.8% stake in the company. Icahn is an activist investor and is trying to get his own people on the board but so far he hasn’t taken any actions to unlock value in the company.

Now what: The excitement over Icahn’s stake didn’t last long as shares trended lower throughout the day. Icahn has a good history of creating value for shareholders but he rarely plans to hold shares long-term, instead choosing quick fixes for companies. I wouldn’t buy just because Icahn did, but if you do think Navistar is a value this can give your position a little more confidence.

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