If you answered C, then perhaps you’re already hyper-aware of the fact that these days, anything and everything just might be some form of an ad. In the case of the cup-on-the-roof, it was a recent stunt designed to promote Starbucks coffee in several cities during the holiday season. The cups—which had strong built-in magnets to keep them from sliding off the roof—were planted on hired cars, which were then driven around town. And whenever any good Samaritans stepped forward to inform the drivers about the cups, they were given five-dollar Starbucks gift cards. But the real payoff came in the form of publicity generated by the stunt, as it garnered attention in local news programs and newspapers.

Starbucks isn’t the only one promoting its coffee with a street-savvy approach these days: Not long ago, Folgers decorated manhole covers of New York sewers so that they looked exactly like hot, steaming cups of coffee, coming right out of the pavement. Want some eggs with that coffee? Last fall, CBS laser-etched ads for its TV shows on 35 million eggshells. Other advertisers of late have managed to sneak their pitches onto banana peels and inside fortune cookies. And Häagen-Dazs recently turned postage stamps into ads for its new ice cream flavors (best part: when you licked the stamp, you tasted the ice cream).

It’s all part of the blossoming of the ad industry’s love affair with unconventional or guerrilla-style advertising. Ads showing up in unexpected places is not necessarily a new phenomenon: The agency Kirshenbaum & Bond was stenciling clever ad messages onto Manhattan sidewalks way back in the late ’80s, and the trend has steadily grown since then. But today’s unconventional advertising is more prevalent and tends to be more grand in size and scope: What was once a sly little message on the sidewalk is now apt to dominate the landscape by taking over an entire building. What’s equally impressive is the ingenuity and creativity often displayed in this category of advertising.

Which is not to suggest the trend is beloved by all. The growing guerrilla ad movement clearly has its detractors, some of whom contend that the continuing spread of ads into unconventional forms and venues—a phenomenon sometimes referred to as “ad creep”—is creating an environment in which there is no refuge from the constant bombardment by ad messages. Aware of the potential consumer backlash against this kind of ubiquitous advertising, some marketers and their ad agencies are striving even harder to make the ads clever, engaging and rewarding—and therefore, they hope, less irritating.

“There is certainly a risk that people will be turned off by ads that seem to be surrounding them,” acknowledges Rob Rich, a creative director in the Seattle office of Publicis West. “But if the ad can somehow contribute something positive—maybe just by making someone laugh as they walk down the street—then I think people are more likely to tolerate it and even enjoy it.”

BBDO creative director Bill Bruce adds: “At its best, this kind of advertising can almost become a kind of street art or a public performance. And when it manages to do that, it doesn’t feel as much like selling.” In its unconventional uses of “wallscapes” on the sides of buildings, BBDO has created the illusion of bugs scaling city skyscrapers (for Jeep Wrangler) and couches climbing the walls (for HBO). The agency has also freshened up the old medium of billboards by adding an interactive touch. A recent Times Square billboard for Mountain Dew was designed so that if you stood at a certain spot on the sidewalk (marked by an X), you became part of the scene in the overhead billboard. This encouraged people to pose and take pictures, using the Mountain Dew ad as a backdrop. And as Bruce notes, “Any time you can get people to engage and spend time with your brand that way, you’ve gone beyond advertising to create a kind of event.”

Rob Schwartz, executive creative director at TBWA\Chiat\Day in Los Angeles, has a similar take on the street-advertising phenomenon. “What we’re seeing with some of this work is a kind of ‘brand theater,’” he says. “It can become an interesting part of urban culture.”

TBWA has, in the past few years, emerged as one of the leaders in the unconventional ad category; in cities world-wide, the agency has been experimenting with dozens of bold new ways to get its messages out to the public. “We tend to see the street as part of the canvas we have to work with,” Schwartz says. For its client Sony PlayStation, TBWA has stenciled highly-stylized graffiti, produced by real graffiti artists, on streets or on sides of buildings. The agency’s work for adidas athletic shoes seems to be reinventing the whole notion of what outdoor advertising is and what it can be: adidas billboards have been morphing into all manner of shapes and forms, practically taking over local landscapes (in Germany, for instance, an adidas outdoor ad takes the shape of a soccer goalie’s gigantic body, stretched across the top of the autobahn).

The agency has also gone further by transforming outdoor ads into live events. In Tokyo, it hosted a live game of “Vertical Football” on a green billboard high above the city streets. Two football players hung sideways, suspended by rope, as they kicked a ball (also suspended by rope) back and forth to each other. The spectacle brought traffic to a standstill and became a news event, not just in Tokyo but worldwide. As Schwartz notes, “People tend to ask, ‘How can any advertiser possibly stand out in a place as crowded as Tokyo?’ Well, this is one way.”

Other agencies have also taken to creating elaborate live events. DDB in London recently introduced a new version of the board game Monopoly by staging a live version of the game, using the streets of London as the board and local taxis as the playing pieces.

Complex, choreographed events such as these can end up being as expensive as making television commercials. But for the most part, guerrilla-style ads still tend to cost advertisers far less than running commercials—which is part of the appeal. According to Jim Haven, co-creative director at Creature, the small Seattle ad agency that dreamed up the Starbucks cup-on-the-car promotion, that campaign cost less than $150,000 to execute (according to Haven, the hardest part was building extra-strong magnets into the cups). The media exposure generated by the stunt—many newspaper stories, as well as TV news coverage—probably surpassed anything that would have been generated by a million-dollar conventional ad buy. In guerrilla ad-speak, this has been dubbed “the multiplier effect”—wherein free media exposure (primarily via news stories) multiplies the investment of the actual stunt. It’s a very appealing prospect for cost-conscious, buzz-seeking marketers, which is why, Haven says, “It now seems like every client who comes to see us immediately asks, ‘Can you do something like that Starbucks cup for us?’”

Cost-effectiveness isn’t the only factor driving the growth of unconventional advertising. Advertisers are finding that many consumers are now harder to reach via conventional ad channels, such as broadcast TV and radio. “Fewer people are watching TV these days, and that’s especially true if you’re trying to reach young, active adults,” TBWA’s Schwartz says. “These people tend to be out on the streets, doing things. The idea is to reach them while they’re out there living their lives.”

With its recent proliferation, unconventional advertising runs the risk of becoming a victim of its own success: The more advertisers try to “reach” people out on the street, the more immune those people become to the messages. “All those outdoor messages can end up blending into one another,” says Rich of Publicis.

That makes it even more imperative that guerrilla advertisers constantly break new ground in terms of coming up with original ideas, new venues and fresh twists that can maintain the element of surprise—which is a key ingredient to successful unconventional ads. Zak Mroueh, executive creative director at the Toronto-based agency TAXI, says: “You have to keep innovating because nontraditional advertising has a way of very quickly becoming traditional. For example, ads in public bathrooms once seemed so different and clever—but now they’re a cliché.”

It doesn’t mean, however, that a bathroom-based guerrilla ad can’t still be effective. Mroueh’s agency recently earned plaudits for a devilishly clever ad for the MINI Cooper car. By installing tiny orange pylons inside urinals, the agency made them look like slalom courses; the headline posted directly above the urinal read: “Test your handling skills.”

While TAXI has relied on humor in the men’s room, Publicis recently took a more pragmatic approach in the women’s room: Using decals of diamond necklaces strategically placed on mirrors, the agency offered a chance for restroom visitors to visually “try on” the necklaces in the mirror to see how they look. In working with another client, Charmin, Publicis found that a bathroom guerrilla ad can actually become a bathroom in and of itself. Charmin has recently begun setting up temporary-yet-elegant public restrooms—stocked with Charmin toilet paper, of course—outside concert venues, as well as in the heart of Times Square, and in other places where people are usually forced to settle for grungy porta-johns or no bathroom at all. The idea, here again, is to bring the ad message out into the world, affording people an opportunity to “experience” the brand as part of their on-the-go lives.

It can be hard to gauge the success of any of these unconventional ads. Whereas conventional advertising has established measurement systems in place, “There’s simply no way to do a pinpoint measurement of guerrilla-type work,” Schwartz says. Indeed, one of the criticisms of guerrilla work is that it’s not held to the same standards as other ads—and that the playful stunts sometimes seem to be produced almost as a lark. But as TAXI’s Mroueh notes, even the most clever stunts are not much good unless they end up being seen by lots of people. “What bothers me is when people create clever or outrageous unconventional ads that are seen by only ten people, then entered in an award show,” Mroueh says. “If you’re going to do nontraditional work, you really should be asking, ‘Who’s going to see this and what is the client going to get out of it?’”

The answer to the latter part of that question is, in a word, buzz. In the end, an unconventional ad should yield “stories in the press, word of mouth, Web chatter,” says TBWA’s Schwartz. “Basically, when you try to gauge the success of this kind of work, you’re looking to see how much the idea was talked about and passed around. Did people deem it to be ‘shareworthy’? That’s the test.” ca