That law has a corollary: The harder the item is to find, the more you will end up paying on the off chance the gift grail is discovered.
In trying economic times, these real world lessons in supply and demand pose difficult and potentially distressing questions.
How much is a holly jolly Christmas worth? Which is more troubling: economic stress or the potential for crestfallen faces on Christmas morning?
Perhaps there's a solution.
When things are hot, they are hot; when they are not, they are often last year's model. Although strikingly similar to the gleaming new model, earlier editions might be easier to find and considerably cheaper. Here's a guide to some alternatives to this year's hot gifts.
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Rock Band 2 ($189.99 for Xbox 360 edition)
Rock Band ($119.99)
The new version of the button-mashing rock band simulation isn't cheap. That's not surprising.
Last year, its predecessor was the budget-busting hit of the season. It's true that there have been a few bells and whistles added to the new version, but for rockers who might have just gotten a console or held off on plastic guitar shredding, the original comes with a smaller price tag.
Blackberry Storm Touch ($199 with two-year contract)
Blackberry Curve ($99.99 with two-year contract)
We're back to those touch screens again. Clearly hot. If you, as a phone user, are willing to press those old buttons, though, savings are available.
Baby Alive Learn to Potty Doll ($48.88)
Gotz Aquini Potty Doll ($26.95)
Sony Cyber-shot DSC-T77 ($299.99)
The Sony is the compact digital point-and-click of the moment, loaded with features and available in a variety of fashion colors. The Finepix has been around for a few years and has fewer gadgets. What it does have is similar picture quality, a sleek design and pretty colors. The Sony has a 4x zoom and the Finepix a 3x, so it will require standing a little closer to photograph subjects.
The Apple iPod Touch 8GB ($229.99)
SanDisk Sansa Fuse 8GB MP3 player ($129.99)
Sure, it's missing the sleek touch screen, but this hardy little unit holds just as much information for a fraction of the price.
Nerf N-Strike Vulcan EBF-25 Blaster ($42.99)
Nerf N-Strike Longshot CS-6 ($29.97)
In terms of mutually assured foam destruction, the Vulcan is clearly the big stick. As the latest and greatest, it demands a premium price, too. The Longshot, a favorite among Nerf warriors, retails for less, and, unlike the Vulcan, can be found at a discount. Amazon lets them go for about $30.