A couple of years ago, some friends bought a vacation house in the mountains. It needed work — e.g., a swath of exterior wall needed to be re-sided where a tenant, using a charcoal grill on the deck, had managed to melt a section of vinyl siding. My theory: Because most people's expectations of exterior walls are formed by long acquaintance with wood, brick and stucco, they just don't expect exterior walls to melt. Even if, on some level, they're aware that they're dealing with walls clad in vinyl siding, which they know to be meltable, it doesn't occur to them to take precautions to avoid melting it.

No doubt some scholar at some university is even now trying to figure out how consumers derive their expectations of manufactured products. How do I know to expect that French bread acquires the consistency of Styrofoam after 24 hours, but artisanal multigrain can last more than a week in the fridge?

Is it just experience? Try biting into a two-day-old piece of French bread and instantly all is revealed. Same way, pull a two-day-old loaf of $5 multigrain out of the breadbox only to find it covered with blue fur, and you relearn the benefits of refrigeration. Probably something about the chemical contents of the two loaves causes them to behave differently, but that's purely a guess. I don't begin to understand what's actually going on. I've just learned to negotiate around it.

Same way, I'm no longer surprised when the treads on my Crocs ballet flats wear off in a matter of months. I've adjusted: I buy them on sale to compensate and try to remember not to wear old ones on rainy days, because the soles are suicidally slippery when wet.

I've been thinking about consumer expectations since last week, when I wrote about the man-made soles of five pairs of shoes that, over the past several years, have turned gooey, or suddenly delaminated, or cracked and begun to crumble, or totally disintegrated over a period of hours. It isn't just me, either. At lunch the other day, an old friend was telling me about a pair of Clarks walking shoes that had seemed perfectly fine when he put them on one morning. A little while later he noticed that chunks of the sole material were falling off. By the end of the day, his shoes had no soles at all.

Google "disintegrating soles" or 'shoes fall apart" and you'll be amazed at how many people have had similar experiences. They're not happy about it, either.

This is — unsurprisingly — not a feature that shoe manufacturers are eager to publicize. So far, I've been unsuccessful in getting any of the companies that manufactured my cracked, crumbled or liquefying soles to tell me how often this happens, or why, or what, if anything, they're doing about it. (Except for the first time it happened, six years ago, when an Ecco brand manager blamed "hydrolysis," which he characterized as "rare.")

The most surprising thing I've learned from my research so far is that the problem of crumbling soles is as likely to be due to defective epistemology — how you know what you know — as to defective manufacturing.

I just read somewhere that a substantial slice of the population believes that our human ancestors lived among the dinosaurs, like on "The Flintstones." And 44 percent think evolution is a lot of hooey; they think people were created 10,000 years ago and haven't changed much since. So how much do you suppose the average shopper understands about the nature of direct-injection-molded polyurethane foam soles? How clearly could he or she explain the differences between PU soles and laminated EVA soles, and how each differs from Neoprene soles?

Forget it. As most of us learn all over again every time we have to buy a new refrigerator or a new computer or a new car, the manufactured goods we buy have long since moved beyond our feeble abilities to understand the science that makes them work. Like primitive tribesman terrified that an eclipsed sun has been stolen by evil gods, we fall back on a mix of faulty assumptions, old saws and magic thinking.

Crumbling soles especially dismay us because, except for cookies, most of the materials we run into don't crumble and suddenly disintegrate. They wear down gradually, then break or tear or collapse. Even fragile materials like glass and china only break if you drop them.

So when the soles of a pair of $250 shoes disintegrate without warning, we assume it must be somebody's fault. And, according to a shoe industry consultant I interviewed, sometimes it is. When a nearly new man-made sole delaminates or disintegrates soon after you buy it, it may be the result of a manufacturing defect — an overheated press, a contaminated batch of material, etc.

The trouble is, injection-molded polyurethane foam also crumbles when it gets old. Get used to it.

Phillip Nutt, the consultant I spoke with, said I shouldn't expect soles made of polyurethane to last more than 4 or 5 years. Think of its limited lifespan as the price you pay for the comfort, support, resilience and shock absorption it delivers. Kind of the way a French baguette is delicious for a day, and a day later good for nothing but packing material. (Or turkey stuffing.)

Nutt couldn't explain why PU soles that are stored in their shoeboxes unworn for months or years at a time seem to degrade even faster. They just do.

On the bright side, he notes drily that polyurethane's propensity to disintegrate when it gets old solves one problem. People used to worry that shoes with synthetic soles would persist in landfills forever because they weren't biodegradable.

Soles that will crumble in a shoebox will crumble in a landfill. Yippee!

Write to Patricia McLaughlin c/o Universal Uclick, 1130 Walnut St., Kansas City, MO 64106 or patsy.mcl@verizon.net.