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 embattled snack maker Diamond Foods
So what: Diamond Foods shares have been under pressure ever since it announced it'd be investigating its own accounting, including looking into payments it made to walnut growers. Since then, shares have been violently whipsawed, posting big gains or further losses based on almost any whiff of news to hit the wires.
Today, rumors swirled that Einhorn -- likely through his Greenlight Capital vehicle -- had taken a stake in Diamond. The rumors started on CNBC, quickly sent the stock spiking, then lost some steam, but left the stock up significantly on the day.
Now what: There are a lot of bad reasons to buy a stock, but high up on that list is buying a stock solely because some other investor bought it -- and that's whether it's David Einhorn, Warren Buffett, John Paulson, or your Uncle Larry. There could be a viable thesis to buy Diamond right now, and perhaps Einhorn's interest (if it's true) could be a good reason to take a closer look at the stock. However, it's important that before you jump in, you do your homework first.
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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.