The problem is that most of the type of questions are "tricks", they require you to come up with a specific insight (or maybe do a little algebra in your head).<p>Asking someone to do this on demand in an interview is iffy; in some jobs I'm sure that sort of puzzle solving is needed, but for most I find it a lot more apropos to ask them to do something related to the job.  E.g. to attack a design problem, where I will indeed be looking at how they work through it.