Not long ago, I sat down with Motley Fool CEO Tom Gardner, who, without realizing it, played the part of summertime Santa Claus.

Like an eccentric playboy tossing money in the air, Tom threw out some of the best and most actionable investing advice that I had ever heard.

And just like any rational person standing next to that eccentric billionaire, I was furiously taking notes so I could pick up everything he tossed out.

Setting a high bar
Of course, Tom's record speaks for itself. As the co-advisor of the Motley Fool Stock Advisor investment service, his recommendations have returned an average of 35% versus 5% for the market since 2002.

So, as an opportunistic learner, I gleaned every bit of applicable investing knowledge I could from that meeting. And boy, the lessons learned have proven most helpful.

What I learned
"Keep it simple, stupid" is an adage that we hear often enough, but rarely implement in our lives. For some reason (good or bad), we humans just love to complicate things. That's fine if you're Faulkner writing The Sound and the Fury, but as an investor, overcrowding the brain can lead to murky results.

The instinct to add complexity where none is needed is precisely the problem -- and it frequently keeps smart investors out of the market. Whether it's constructing highly detailed financial models or constantly adding new elements to the investing thesis, an unneeded complexity throws the baby out with the bathwater -- and it simply gets in the way.

As the great French writer and aviator, Antoine de Saint-Exupery, said, "A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." The same is true of investing.

3 rules
Keeping it simple, Tom asks three straightforward questions when evaluating an investment. Here they are:

1. Is this company delighting the customer? Look at what Research In Motion (NASDAQ:RIMM) has done for the corporate consumer and corporate productivity. Most people need coffee in the morning, but not everyone needs to spend $5 on it -- that's why Starbucks (NASDAQ:SBUX) focused on pleasing its patrons first and, in doing so, has changed the coffee industry. Your investments should be doing everything possible to make every customer a repeat customer.

2. Are the fundamental economics of this business beautiful? Unfortunately, some companies will go to hell and back trying to please their customers -- but they'll do so in vain because the underlying economics of their business are just not good enough. JetBlue (NASDAQ:JBLU) is a great example. Airlines are a notoriously difficult industry in which to make money. I'd venture to say that Whole Foods (NASDAQ:WFMI) is facing similar difficulties in the low-margin grocery business. No matter what a company does to please customers, the potential to consistently make a healthy profit must also be there.

3. Does this company have room to grow? I think we all can agree that Coca-Cola (NYSE:KO) delights it customers and runs a fundamentally attractive business. But Coca-Cola has already had a tremendous run. That's not to say you won't earn a healthy return by investing today -- but trees can't grow to the sky and Coca-Cola can only get so much bigger than it already is. This stands in contrast to a company like Under Armour (NYSE:UA), which can say yes to questions 1, 2, and 3 -- and that's where Tom sees real potential.

Foolish bottom line
Is this too simple a screen? Maybe. Does Tom ask more of his investments than these three questions? I'm sure he does. But if you can categorically answer "yes" to each of these questions about a potential investment, you're off to a great start -- because those are the great businesses.

You can ask these questions of your own investments -- and you should -- but if you'd like to see which companies Tom says yes to, then look no further than The Motley Fool's Stock Advisor service. You'll find out why Morningstar (NASDAQ:MORN) can roundly answer "yes" to Tom's questions -- and it's a big reason why we're so darn excited about it. You can get a free, 30-day trial to the service -- with no obligation to subscribe -- by clicking here.

Nick Kapur owns shares of Morningstar. Whole Foods, Morningstar, and Starbucks are Stock Advisor recommendations. Coca-Cola and Starbucks are Inside Value choices. Under Armour is a Motley Fool Hidden Gems pick and a Rule Breakers pick. The Motley Fool owns shares of Starbucks, Under Armour, and Morningstar. The Fool says yes to its disclosure policy.