Sometimes it seems like Mr. Market simply woke up on the wrong side of bed. Today is one of those days. After sinking this morning, the Dow Jones Industrial Average (^DJI 0.61%) is up by 0.84% as of 1:55 p.m. EST.

Perennial loser Hewlett-Packard (HPQ -0.11%) sports one of the Dow's largest gains today, jumping 4.4% on nothing but fluff and dreams. Speaking at a conference in Germany, CEO Meg Whitman said the Autonomy software business isn't all dead weight and might actually contribute to HP's sales at some point. That's all it took to make HP a big winner.

On the other hand, eternal winner Apple (AAPL -0.25%) plunged 4.7%, shaving some $23 billion off Cupertino's enormous market cap. To put that drop into perspective, HP is worth $27.6 billion right now, or just 20% more than the shareholder value Apple burned today.

And why, pray tell, did Cupertino slump so swiftly? The short answer is that there is no obvious reason.

Some market watchers blame reports that Google (GOOGL 10.35%) and friends are stealing Apple's thunder in the tablet computer space. OK, fine, but they're doing it by growing their own slice of the pie -- not necessarily by stealing anything from Apple's plate. Nobody needs to die here.

Others point to the fiscal cliff, speculating that Apple investors might be selling before the new year, when capital-gains taxes may or may not jump. After all, the stock has still gained 40% over the last year, or 200% in five years. But then, why aren't recent market darlings dropping like flies all over the map today? The same tax calculations would apply equally to longtime Apple partner Skyworks Solutions (SWKS 1.40%), for example -- the two stocks have climbed in virtual lockstep for years:

AAPL Chart

AAPL data by YCharts.

And yet Skyworks gained 0.5% today, watching Apple burn from a distance.

Let's just face it: The stock market doesn't always make sense. No news can be good news -- or bad. Game-changing information can move stock prices in the wrong direction. We just have to accept the crazy volatility, buy on the stomach-wrenching dips, and sell at insane, unsupported highs.

Luck favors the opportunistic, after all.