Q: What’s the difference between a guitar and a fish?

Q: What’s the difference between a guitar and a fish?
A: You can’t tuna fish.

The reason that this is funny is a pun. You can tune a guitar, but a fish is not able to be tuned. There is, however, a fish named a tuna, and it is possible to fish for tuna, so even if a fish cannot be tuned, it is possible to tuna fish.

So, the answer, "you can't tuna fish" is wrong, but the answer "you can't tune a fish" is correct. The ambiguity between the two of the options is where the humor lies.

I don't really like this joke. It isn't that funny, but the pun is excellent. I am not quite sure why this might be the case.