Well, let's compare "what is the speed of sound" and "what is a for loop". Both are domain-specific questions, one for physics, the other for programming. For both I'd accept an answer that describes the notion or idea behind the concept and not a memorised instance of it.
So, for "what is the speed of sound", I would accept an answer such as "The speed of sound is the speed at which a wave propagates through a given medium"; though I expect an actual physicist to involve molecules, springs and so on in his answer. What I would not accept is a string of digits. A string of digits shows you know how to remember a string of digits.
Similarly, for "what is a for loop", an acceptable answer is one such as "A for loop is a construct for bounded (at least in principle, but you can have unbounded for loops in some languages) iteration over a series of elements, either generated on-the-fly or from a concrete container". The analogue to a string of digits for this question would be to give the BNF definition of a for loop in C. I think you'll agree that knowing C syntax doesn't show you know how to program.
Given two candidates (for a C programming position), I would of course prefer those who know the syntax. Both because that's one less thing they need to learn before they can become productive, and because knowing C's syntax is indicative of spending a lot of time programming in C.
So to return to your comparison, if I were hiring for a position that required working math where the speed of sound was required... I would treat knowing that constant by heart indicative of their knowing the rest of that subject matter by heart. I'd want to test a lot of that as well. "What is the speed of sound?" would be like a fizzbuzz.
But any reasonable person should be able to answer the speed of sound question. After all, the rule of thumb for estimating lightning distance is that every 5 seconds is 1 mile. Reciting a definition is no better than reciting a string of digits.
> The speed of sound is the speed at which a wave propagates through a given medium
That's really the only 100% correct answer as well, since the speed of sound varies by temperature, humidity and a lesser extent, atmospheric pressure. If Edison expected a discrete number, that would be kind of disappointing.
So, for "what is the speed of sound", I would accept an answer such as "The speed of sound is the speed at which a wave propagates through a given medium"; though I expect an actual physicist to involve molecules, springs and so on in his answer. What I would not accept is a string of digits. A string of digits shows you know how to remember a string of digits.
Similarly, for "what is a for loop", an acceptable answer is one such as "A for loop is a construct for bounded (at least in principle, but you can have unbounded for loops in some languages) iteration over a series of elements, either generated on-the-fly or from a concrete container". The analogue to a string of digits for this question would be to give the BNF definition of a for loop in C. I think you'll agree that knowing C syntax doesn't show you know how to program.