Use of comma with 'which' in this sentence

Question

I would like a ask a very small question -

He asked a riddle which I was unable to solve.

I am confused whether I would put a comma before 'which' or not. Putting a comma before which sounds ok and in fact, the sentece looks good, but, not using comma doesn't make it absurd either. What I am asking is, should I write -

He asked me a riddle, which I was unable to solve. or He asked me a riddle which I was unable to solve.

The first one sounds better to me, however I am unsure of any rule which supports my opinion.

Answer 1

Whether or not a comma is placed before the relative pronoun which matters. That comma actually changes the meaning of the sentence.

1. He asked a riddle which I was unable to solve.

2. He asked a riddle, which I was unable to solve.

In the first sentence, leaving out the comma suggests that he asked several riddles and therefore must explain to you that we are talking about a particular riddle -- the one that couldn't be solved.

In the sentence #2 placing the comma before which suggests that he asked only one riddle -- the one you couldn't solve.

Merriam Webster tells us:

That, which: Although some handbooks say otherwise, that and which are both regularly used to introduce restrictive clauses in edited prose. Which is also used to introduce nonrestrictive clauses. That was formerly used to introduce nonrestrictive clauses; such use is virtually nonexistent in present-day edited prose, though it may occasionally be found in poetry.

Answer 2

In your example case, the meanings are the same, it's basically stylistic.