Google (GOOG) triples fourth quarter profits -- and its stock tumbles $20, or about 4%. Why? MarketWatch says:

The company's gross profit margin also fell slightly, however, as Google increased investment in new businesses. Also, Google reported that the amount it pays affiliates and partners rose during its fourth-quarter, and will continue to do so in the future. Margin and cost concerns contributed to a premarket decline in Google shares. The dour investor sentiment spilled over to Thursday's opening moments, when Google shares fell 2.8%.

Seems plausible, right? We could speculate all day. (Here's theStreet.com's take.) But Fortune's own editor, Andy Serwer, has a simpler explanation:

Luv the buy on the rumor sell on the news on Google. Dang stock up $7 to $501. Then earnings come out...nearly triple! And the stock's off $9 in the aftermarket. Newsflash: It's not over. [It's down $20 as of 11 a.m.]

Seems to be a pattern lately. It's always good to remember that the market isn't a thinking individual, with reasons for what it does, but just a place where people speculate for a living. Andy's right that it's hard to read much into what it does on a given day.

[UPDATE: Alex in New York comments: "Is this morning's selling institutional or individual?" That's a good question. Is one person dumping out, or are just lots of big traders -- like mutual funds, pensions, or hedgies -- realizing their gains? It's hard to answer that question on the day of trading if you're not on the floor. Hopefully someone's recap tomorrow might have that answer. Otherwise, check your Bloomberg terminal or Yahoo Finance next week to see if they posted any big movements in major shareholders. There's generally a lag to posting stuff like that.

Also of interest: Google was actually upgraded today by eight analysts. Unlike Apple, which was downgraded by some analysts, including J.P. Morgan (which said "our view on Mac share gains has proven to be too optimistic") on the day it announced a great first quarter. This further points to an un-meaningful drop in GOOG's shares today.]