Perhaps you've seen those pop-up books -- the children's picture books in which cut-outs of characters and scenes fold out in front of you as you open each page.

Or pop-up ads -- those annoying advertisements that flash into the middle of the Web site you're trying to read on the Internet.

These days, one never knows where a surprise store will pop up -- or what product it will be hyping.

There are pop-up toys, too, like jack-in-the-boxes, in which a clown on a spring leaps up as you open the container.

And now there's a hot, new pop-up phenomenon. It's called the pop-up store.

Many of us remember the days when a vegetable peddler or knife-sharpener would drive his truck slowly down the street. He'd ring his bell, stop for customers, and move on. And there were the patent-medicine hucksters who would pull into town, set up a tent, hawk their high-alcohol potions, and be gone overnight.

They all helped inspire the idea of opening a store for just a week or a month or so, creating a buzz among customers. That buzz, you see -- the promotional impact of this disappearing act -- is the point of it all. It gets people talking and creates a demand. Before you know it, consumers are searching high and low for your product.

The giant discount retailer Target often sets up temporary stores to showcase products.  It even opened a floating store on the Hudson River one Christmas season

In Chicago, for instance, Kraft Foods rented an empty store on trendy Michigan Avenue and began passing out -- giving away! -- slices of DiGiorno pizza. Delta Airlines' discount carrier, called "Song," opened a store in York City's trendy SoHo District, then closed it a week later. The Kodak camera company kept a New York gallery open three weeks, just so folks could test its new photo scanners.

Did we tell you about the women's-fashion pop-up store in New York's Rockefeller Center?

Sorry. Too late. You missed it!