Plural possessive

Though the singular possessive is also correct, I would be inclined to write frogs' eggs, unless it was somehow necessary to identify a particular frog.

Opinion subject to croak, of course.

frog eggs - no possessive needed.

fish filets

That too

Ok thanks guys. I was rather hoping there was a right way and a wrong way, and that they would both involve apostrophes.

I'm writing a story and it kind of hinges on one of the main characters making a subtle punctuation error, and I would like this error to be right at the heart of the topic under discussion. Seems like I should think of something else - "frog egg" does seem right now you've said it.

Ok! Back to the drawing board! (Any suggestions welcome )

If it's dialog, well, spoken English isn't really punctuated. Problem solved?

No, it's a written mistake in the story. In any case, it needs to be *some sort of* mistake because it needs to act as a clue. If I didn't have a mistake it wouldn't be solved - in fact, that would be the problem.

Are you looking for a not so common error, but one that should have been known by the character? Like a person impersonating an a2k member not knowing what a hamster is?

My two cents' worth is that they must be childrens' stories.

http://www.grammarbook.com/punctuation/apostro.asp

@usery - yes, children's story. 6-10yo. I'll check that link for ideas - thanks.

@neologist - so because it's a children's story aimed at 6-10 years it should be something that is being learnt at that age. Not obscure so the kids have no chance of catching it, but not so easy that it seems an incredible mistake to make.

I'm currently considering changing to a spelling error, rather than a punctuation error. That would be easy to work in to the dialogue as it's already written, I just have to think about the word and make sure it's central enough to the discussion to bear repetition while being correctly targeted at that age range (tricky).