You never know where your next investing insight will come from. My latest brainstorm, via an article at InsideHigherEd.com, came from college cafeteria trays.

Colleges have discovered that when their cafeterias don't offer trays to student diners, the students waste less food and drink, and often eat less, too. The article cited an example from New York's Alfred University:

Students ran a test last semester showing that on two days when trays weren't offered, food and beverage waste dropped between 30% and 50%, according to Kathy Woughter, vice president for student affairs at Alfred. That amounts to about 1,000 pounds of solid waste and 112 gallons of liquid waste saved on a weekly basis, according to the college.

It makes sense, no? Imagine being in line at your alma mater's cafeteria. If you've got a tray in your hands, you'll likely grab whatever foods interest you. You might also fill several glasses with beverages, lest you end up thirsty mid-meal. You'll eat as much as you can or want to, and you'll send the tray, still with food and drink on it, down the chute to the dishwashing room.

But if you don't have a tray, you'll be a lot more judicious. You'll choose the foods and drink you really want. You might make an extra trip to hunt and gather a little more, but you'll limit how much you take.

The investing parallel
What does that have to do with investing? Think about your portfolio as a big tray. How many different stocks can it hold? A heck of a lot! Hundreds, if you want it to. Too many holdings can be a problem for investors. 

For one thing, if you own a lot of stocks, you can't give each holding much attention. With 100 stocks, will you really be able to check out each quarterly and annual report? Will you be able to follow the company's developments in the press? (I didn't think so.)

Here in Fooldom, we've long written about the benefits of "concentrating" or "focusing" your portfolio. After all, imagine that you're interested in 100 companies. Try roughly ranking them according to how exciting and promising they are to you. Is it as effective to have money in your 85th-best idea and your 97th-best idea, instead of in your top 10 or 20 ideas? Where will that money be most likely to grow quickly? In your best ideas.

While I truly believe that, I'll confess that I don't always heed my own advice. I now own stock in more companies than I can follow comfortably. That's not the kiss of death, though. After all, broad index funds, which tend to hold stock in hundreds or thousands of companies, have routinely outperformed most managed mutual funds sporting far narrower portfolios.

Still, I believe in the wisdom of investors such as Warren Buffett, who has spoken repeatedly about the perils of overdiversifying one's holdings. Investors like him, who aim to focus their investments, have a good shot at doing very well.

More investing implications
There are inverse implications from the cafeteria findings, too. My suspicions were confirmed at designboom.com, where I learned that over time, shopping carts have increased in size, as retailers discovered that bigger carts led shoppers to buy more.

Those huge carts at Costco (Nasdaq: COST) and Wal-Mart's (NYSE: WMT) Sam's Club are no coincidence. Even at more traditional retailers such as Target (NYSE: TGT), and supermarkets such as Kroger (NYSE: KR) and Safeway (NYSE: SWY), odds are good that someone's paying close attention to shopping-cart size.

Finally, here's a last lesson for us all: Pay attention to the size of your containers when you're shopping or adding to your belongings. If you have massive bookcases, you'll likely fill them up. If they're small, you may focus on owning just the books you love. If you go to a store and are greeted by Nebraska-sized shopping-carts, don't let yourself buy more than you planned to.

Whatever you're reading or seeing or thinking about, odds are you can draw a useful financial lesson out of it. Try it!

You can learn more about concentrated investing -- including some points against it -- in these articles:

Longtime Fool contributor Selena Maranjian owns shares of Costco and Wal-Mart. Wal-Mart is a Motley Fool Inside Value recommendation, and Costco is a Stock Advisor recommendation. Try our investing services free for 30 days. The Motley Fool is Fools writing for Fools.