According to a recent survey posted by Glassdoor, a jobs and recruiting site, more and more companies are throwing curveballs to candidates—interview questions designed to stump people. We’ll offer some samples here, but we began by seeking the views of Patricia Siderius on this trend and interviews in general.

Each company is unique. Siderius is managing director of executive services for BPI group, a global HR consulting firm, working in its Chicago office. In her role of helping executives to change jobs and/or careers, she counsels them in interview techniques—and hears about their experiences with recruiters. Clearly, she finds, companies are upping the ante in seeking the candidates they want.

One reason she hears of is that many industries are undergoing huge changes in the skill sets they need in employees. But other observers believe such changes have been exaggerated and that the real reasons employers are so picky are two: In this economy, they’re reluctant to hire, so they seek the "perfect" candidate for every opening. Second, they can afford to be choosy, given the unusually high number of jobseekers.

Whatever the reasons, here are some sample questions that jobseekers shared with Glassdoor, and which companies asked them: (1) "What value-added tax is applied to each of these products—tomatoes, T-shirts, and cars?" (consulting firm McKinsey & Co., of candidates for junior consultant); (2) "How many people would use a drug that prevents baldness?" (Boston Consulting Group, of candidates for associate); "What’s the marginal cost of a gigabyte in gmail?" (Google, of candidates for associate product manager).

Siderius suggests that the firms posing these questions may truly believe the queries get at the in-depth knowledge required of employees in such positions. She adds that each company that creates such questions might offer a different rationale for why they do, with the technology companies saying they’re seeking creativity and edginess, for example.

 She contrasts such questions with those she considers truly bizarre: (1) "How long will it take to move Pike’s Peak?"; (2) "If you could be an animal, what would you be?"; and (3) "Why are manhole covers round?" Companies, or individual recruiters who ask such questions, she suspects, think they’re trendy but have long since lost track of what the answers will reveal about candidates. Little wonder, then, that she discourages them.

But what are the real problems with interviews? Siderius trusts that when companies initially design what we might call their overview questions—those they plan to ask of candidates for most jobs—they will do it right. They’ll identify their organization’s values and culture: Who will have the right attitude to work here? What strengths do most jobs require? Questions after that should depend, she advises, on the particular job description governing the spot recruiters are trying to fill. And, they will aim to uncover whether applicants’ knowledge base and experience are appropriate for the opening and whether they have the right work ethic.

At that stage, she recommends, decide whether testing will be required and, if so, whether they’ll focus on skills or behaviors, or both. Let’s assume an organization has done its homework properly and planned the questions for interviews for most expected openings. Why can things go wrong after that? Siderius believes two problems can derail the process and make it ineffective.

First, supervisors and other interviewers have been poorly trained—or not trained at all—in conducting interviews properly. Second, the organization has failed to devote enough resources to the process. She advocates that a team of at least three people—such as the hiring manager, someone from HR, and an employee who’s successful at the job in question—should evaluate each candidate. A lone interviewer can have blind spots, she notes.

Siderius says, "Interviewing is costly and should be considered an investment and an important project that, if done well, will be productive for the company for years to come." She adds that respect for and follow-up notification to every interviewee is crucial to a company’s reputation in the long run. A final note: Avoid the question about manhole covers!