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 Kronos Worldwide (NYSE: KRO) briefly plummeted more than 16% after reporting strong first-quarter results. The stock has since recovered to down 6%.

So what: This sell-off mystifies me. Kronos, whose titanium dioxide compound is used to brighten everyday products, reported $420.4 million in revenue, up 31.5%, and $1.04 in per share profit, up 19.5%. Wall Street was expecting $400.09 million and $0.79, respectively, according to Yahoo! Finance data.

Now what: What gives? I wish I knew. But maybe it doesn't matter. For long-term investors, today's sell-off could be just the buying opportunity they were seeking. Unlike direct peer Huntsman Corp. (NYSE: HUN), Kronos now trades for a discount to its estimated long-term growth rate -- resulting in a perfectly reasonable 0.72 PEG ratio.

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