What makes a stock go up? Short term, a stock's price can be lifted by more things than you can shake a stick at. Long term, it's valuation that matters. Today, I want to take a brief stroll through a few things that make a stock "go up." I'll close with the best reason -- the one we focus on when seeking out Hidden Gems for our premier small-cap newsletter of the same name.

But first, the fun stuff: the "sex and lies" promised in that catchy headline you see above.

Sex
As the old marketing saw goes, "Sex sells." And it's as true for stocks as it is for toothpaste. Some stocks are just downright sexy, no two ways about it. It's the reason their stock prices go up and up, regardless of whether the business behind the stock -- and the numbers behind the business -- support the company's lofty share price.

Take stun gun manufacturer Taser (NASDAQ:TASR), for example. The company makes the closest thing you'll find to a real, live ray gun, this side of a Star Trek rerun on UPN. Take the big-bang appeal of a gun, combine it with boys' (and men will be boys) eternal fascination for everything sci-fi, and you've got yourself one sexy stock, practically guaranteed to go up. Is it attractively priced? Who cares!? Zap! Ouch! And up goes the stock!

Until it goes down
Though you ordinarily wouldn't think of doughnuts as "sexy," back in its day, Krispy Kreme's (NYSE:KKD) krullers had all the curves. For years, it seemed the company could do no wrong -- until one not-so-fine day, it suddenly could do no right. And that's the danger with sexy stocks. Like rock stars, they can be awfully popular for an awfully long time, until one day the wrinkles appear, the love handles come on, or the star gets caught lip-synching. It takes only one public goof to turn a sex object into an object of scorn and derision.

Lies
Some companies seem to have the magic touch. Their sales increase exponentially. Profits go through the roof. Wall Street analysts rave. These stocks just can do no wrong. Except for one thing: It's all a lie. You know which ones we're talking about: companies that make funny money by stir-frying the bejeezus out of their books. Companies with names like Tyco (NYSE:TYC), Adelphia (Pink sheets: ADELQ), HealthSouth (Pink sheets: HLSH), and Enron.

There's a reason the good Lord listed these stocks on the NYSE for us, Fools. It was to teach us the moral of the story: If it sounds too good to be true, it probably is.

And the lying liars who tell them
And then there are the hype jobs. We're not talking about the book cookers this time, or not just them. We're talking about companies that might have quietly lived out their lives in penny-stock land, then just as quietly expired in solitude and anonymity. We're talking about the twin yo-yos of satellite radio: Sirius (NASDAQ:SIRI) and XM (NASDAQ:XMSR). We're talking about the profitless wonders that produce little more than PR and pay their CEOs more than they deliver in annual revenue, such as Research Frontiers (NASDAQ:REFR).

Or look at the latest poster child for unrealistic expectations, posted hither and yon across the "discussion" boards on Yahoo!: Travelzoo (NASDAQ:TZOO), a company whose ratio of enterprise value to free cash flow is so sky-high that predictions you read about it no longer make you bat an eye. There's no arguing the stock hasn't been a good investment. But, heck, a lottery ticket can be a good investment. You just can't know that it was, until it has paid off. "Investing" in hype jobs is about as reasonable as "investing" in lottery tickets. You may get lucky -- once. (If you do, then count your blessings and hide your winnings.) But play too often, and, ultimately, you'll lose everything -- down to and including the dollar you started with.

Why stocks should go up
As Warren Buffett -- paraphrasing Ben Graham -- so elegantly put it: "In the short term, the market is a popularity contest; in the long term, it is a weighing machine." One thing that all of the above scenarios have in common is this: The stocks that go up are popular. Raise your hand if you haven't heard of Enron. How about Krispy Kreme? Thought so.

Now, how many of you have heard of a little business called Mine Safety Appliances (NYSE:MSA)? For those who haven't, here's a quick rundown. The company was Tom Gardner's third recommendation for Motley Fool Hidden Gems, way back in September 2003. Practically no one on Wall Street knew it existed then. Today, next to no one does. There's a grand total of five analysts following the company. Compare that with the 36 analysts following Intel (NASDAQ:INTC), or the 39 following Cisco Systems (NASDAQ:CSCO).

You really have to wonder: Why all the interest in a couple of stocks that have, over the past year, underperformed the S&P 500 by double-digit margins? Why so little interest in Mine Safety, which has blown out the S&P by 91% on its way to yielding triple-digit returns over the past year -- and returns greater than 200% since Tom's recommendation 15 months ago?

It's quite simple, really. You'd have to think long and hard to come up with a name less sexy than "Mine Safety." Yet that very lack of sex appeal is what kept analysts from noticing that over the past five years, Mine Safety has increased its earnings an average 26.2% per annum. For all Cisco's popularity on Wall Street, its earnings grew at a rate less than half that. Intel has seen earnings decline an average 19.1% every year for the past five years.

So, yes, sex may sell. But at Hidden Gems, we'd rather buy performance.

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Fool contributor Rich Smith has no position in any company mentioned in this article. The Motley Fool is investors writing for investors.