"Imagine that I had a drinking cup full of water. Now imagine that next to that cup, I had a second identical cup filled with a liquid that looked absolutely identical, but actually was completely inviscid. Design a test to discern which liquid was which."

"I don't understand the point of the question, because zero viscosity is impossible and therefore the question has no practical relevance."

"Well, you could just drink both cups."

what the hypothesis was behind this test.

"I would rotate both cups."

"Pour a few drops of liquid onto the table. The water will bead up, while the inviscid liquid will spread out completely since it has no surface tension."

"Place a drop of food coloring dye into each cup. The drop spreads by diffusion, which is limited by the viscosity of the liquid. Therefore, the drop in the inviscid liquid should spread to fill the whole cup much faster than the drop in the water."

"Well, once you hire me, I can tell which liquid is which because I won't be able to work with the inviscid liquid. The pipettor relies on surface tension to pick up liquid, so I won't be able to get the pipettor to work with on the inviscid liquid."

In my career, I've gotten to do a fair amount of interviewing for both engineering and biological science (e.g. biochemistry) positions. Based on some things I picked up in grad school, I liked, late in the interview, to ask a pretty tough question to all candidates:The point of this question is to get at the candidate's problem-solving abilities, as well as their ability to design experiments. Given the unusual nature of this question (specifically the hypothetical existence of a theoretically impossible physical property), only the rarest of candidates would have an immediate answer. I liked to give the candidate time to think, and to see how he or she would approach the question.The most popular thing coming first out of any given candidate's mouth was, "What does 'inviscid' mean?" In my mind, an engineering candidate would lose a bit (and only a bit) of merit if this question was asked, since anyone who has had basic fluid mechanics should know that an inviscid fluid is one with zero viscosity. I wouldn't expect biochemistry majors to necessarily know this. In practice, only about 25% of candidates would know what 'inviscid' meant (although, to date, every single candidate who knew the definition double-checked with me that I was, in fact, referring to a zero-viscosity liquid.)Following are a series of responses I'd get after that typical first one, along with my impressions of the candidate.This particular response bugged me. On the one hand, it is a valid point. For some candidates, this response may represent a strong rooting in problem-solving by wanting to address only practical and relevant issues. However, I feel that a job interview is the wrong time to call into question the relevance of a technical issue. This question (or really a comment) would be better saved until the end of the interview, when the candidate could ask something like, "Why did you choose to ask a question that was not technically possible?" I have, in fact, been asked this at the end of interviews, when I would explain that the goal was to see how the candidate problem-solved out of the box with a topic I could be almost entirely sure they had never previously considered.I got this response a lot. In fact, it was usually a "gut response", from about a third of the candidates. It was almost always said with a little laugh, a sure sign that the speaker is not confident in what he or she is saying. (Notice how often President Bush laughs when he speaks.) My response to this statement was always to askAt this point, the candidate will do one of two things: GOOD = to say "well, actually I'm not sure how you could tell from that...let me think some more." BAD = "well, I'm sure you could tell somehow." If they made the GOOD response, I'd chalk off their initial response to answering too quickly, and I'd wait for a real response after they thought some more. If they made the BAD response, I'd begin badgering them, asking WHAT measurable thing would happen differently if they drank an inviscid liquid. Rarely would I get a good response. By this point, you already know that this candidate does not understand the basics of experimental design and they do not know how to develop and test hypotheses. You already know that if you hire this person, he or she will require explicit instructions on everything to be done, and otherwise will do everything by basic, inefficient brute force.(Just as an aside: what would actually happen if you drank an inviscid liquid? I'm not entirely sure, except that you might have trouble not spilling it due to the absence of surface tension as it exits the cup and touches your lips. I think it's also possible that the liquid would slide down your throat without swallowing, again due to the absence of surface tension.)When I asked this interview question, what I really wanted to see was the candidate come up with a basic test that was measurable in some way. If it took them some time to get there, I didn't mind.So, one response went like this:So I ask how they would behave differently and what could be observed to determine which liquid was which. The candidate said something I really liked which was, "I haven't figured that out yet, although I am pretty sure this will work. Just give me a minute." So we waited, for about a full minute. Then she said, "OK--if you rotate the cup containing water, the water inside will begin to swirl in the same direction as the cup, but I think the inviscid liquid would not swirl...is that right?"So the candidate wasn't totally sure, but was absolutely correct. I really liked this response for several reasons: 1) The candidate picked an area where she was pretty sure the viscosity was going to be relevant, meaning she had at least a basic skill for narrowing down a broad problem using basic principles 2) The candidate was not afraid to sit in silence and take the time to think about the problem, plus not to mention the fact that she thought well under pressure--it's not like that's the most comfortable situation and 3) She wasn't afraid to represent a partial theory she wasn't entirely sure of, and to properly represent her level of conviction. This is just what you want to see someone bring to a team: present ideas, state their case as best as they can, and be open about their level of confidence. This candidate demonstrated to me at least a good starter level in problem solving skills, as well as at least a good starter level in communication skills and honesty. (She was hired.)There are a whole bunch of other answers that are similar to the above one, in that they revolve around very basic tests. Another example:What an awesome answer. This guy gave the experiment, the expected result, and the scientific reasoning all in one complete answer. He gave me confidence in his ability to design experiments, as well as his problem-solving and communication skills, all in one-fell swoop.Ditto on this answer as compared to the one above. The woman who gave this answer ended up with a job offer.One candidate was being interviewed for a position that involved a lot of engineering on automated pipettors (for picking up and dispensing liquids in automated fashion.) He answered:Ha! This guy was brazen, but he really impressed me. A great answer, just like those above. But he did two other things I liked: 1) He related the question directly to the job description he was interviewing for, which shows excellent problem-solving skills by applying the knowledge at-hand to the problem and 2) He asserted himself as a candidate by suggesting that he was already getting the job. That's cocky, but it also shows strong self-confidence and ambition. Those qualities are difficult not to like, and indeed this guy was hired.Keep in mind that this was just one interview question among many, and I was only one interviewer among many. The way the candidate handled this question was just one data point towards the full evaluation. However, I'd estimate that about 90% of the time, this question revealed the candidate's true level of problem-solving abilities