What's the marginal cost of a gigabyte in gmail? If you know the answer, you may have a shot at becoming an associate product manager at Google. If you don't, hey, don't feel too bad. Google ranked 9th among the 25 most difficult companies to interview with. In fact, of those 25 , a dozen are tech companies. according to a report by Glassdoor. Google scored 3.5 out of 5 on the difficulty scale of 1 to 5, while Facebook and Amazon weren't far behind -- each has a 3.3 rating.

Why Tech is Tough

"We're finding tech companies are moving very quickly and they need to find very skilled workers," says Scott Dobroski, a Glassdoor spokesman and community expert. Indeed, Dobroski says company recruiters aren't waiting for the perfect candidate to come through the door. They're also proactively scouring social media networks looking for the right candidates. He also says that tech companies ask candidates for both IT and business roles questions like, "What would you change with the company's products and how would you push them forward?" And beyond questions about your hard skills, they'll also dive deep into your thinking process. For example, a candidate at Unisys was asked if "skill or will was more necessary" to get the job done. At Google, a product manager candidate was asked how many hotels are in the United States. A systems administrator candidate at Rackspace had to conjure up as many uses as they could for a brick, in one minute. "These questions are asked to test your critical thinking," explains Dobroski. "The worst answer you could give is 'I don't know.' You should sound out your response as you think your answer through. They want to see how you think on your feet, how you dream and what solutions you come up with on the spot." Hmm...those oddball questions don't seem so odd anymore, eh? "I find many times companies ask these questions and have extensive interviews to assess culture fit," says David Chie, an executive with recruiting firm Palo Alto Staffing Technology. "Most of these technology companies have experienced explosive growth and the leaders realize that the internal culture is a huge component in their success or failure." He added these firms also usually have a high level of applicants, so they feel they can be more selective in their process.  Other firms don't always have that luxury.

Here's a look at Glassdoor's Top 25:

Glassdoor rated these companies based on culling more than 80,000 interview reviews over the past 12 months. Each company had to have at least 20 interview reviews and 20 company reviews to be included.

Tough Love

"What unifies almost all of these companies is that they're hugely successful and have dozens (maybe hundreds) of applicants for every opening," says Jon Holman, president of executive recruiting firm The Holman Group. "All their applicants have high SAT scores and great grades, so they need some way to pick only the best and brightest.  Their answer is to ask these hard questions." Microsoft started this trend among the technology companies, Holman notes, pointing to a book about their practice called "How Would You Move Mount Fuji? Microsoft's Cult of the Puzzle - How the World's Smartest Company Selects the Most Creative Thinkers." "Years ago, there used to be companies that deliberately conducted stressful interviews, not by the difficulty of the questions and the creativity required for the answers, but by actually stressing people with physical conditions or semi-abusive interviewers," Holman recalls. "This is a whole different kind of difficulty, maybe no less stressful but more intellectual and not masochistic." And while these tough questions are designed to test job applicant's creativity and fast thinking skills, companies usually find most people only possess one or the other - not both, Holman notes. "Unfortunately, (hiring managers) tend not to hire deeply creative people who require time to think, and thus may miss the true innovators, but it's hard to interview people who want to get back to you tomorrow with the answer," says Holman.