[mseebach]

The premise of the question is that a pen is ubiquitous and used all the time by everybody. You can trust that the interviewee already understands exactly what a pen is and does (as opposed to the company's niche Foo Widget -- in this way it's similar to the classic datastructure/algorithm coding question: no, you'd never actually be solving this exact problem in the job, but it's a level playing ground to see if you can display some qualities we expect to be correlated with things you need to do in the job). If that premise fails, you (as an interviewer) need to come up with another object that does meet the criteria (say, an iPhone).

But if the interviewer is trying to have an interviewee sell them a product they up front know they're not even mildly interested in buying (as is probably the case for most of the HN crowd, we'd probably never actually buy a pen - digital for almost everything, free pens from wherever for whatever might need writing), they're either cynically hiring for a boiler room operation or doing it wrong.