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: Mr. Market sneezed on Thursday, but MGM Resorts (NYSE: MGM) caught pneumonia and died. By close of trading, the stock had dropped 13%.

So what: Just another case of a high-beta stock being more volatile than the rest of the market? You wish. There was actually a reason for MGM's fall -- strategic investor Kirk Kerkorian announced today that he's selling 20 million shares of MGM, flooding the market with unwanted MGM stock -- which promptly plunged.

Now what: Are investors right to panic? In a word: no. Even 20 million shares isn't all that much to Kerkorian. According to a regulatory filing, this massive sale represents only about 15% of his stake in the company. In short, Kerkorian is not bailing out of MGM. To the contrary, he still owns more than 111 million shares of MGM stock.

In other words, if you liked MGM before today's sell-off, there's no reason not to love it at a 13% discount.

Want to learn more about MGM Resorts? Add it to your Fool Watchlist.