Story ideas

People who offer story ideas to you "for a small fee" don't understand that they're a bit like kittens on a country farm — it's not that they're unwanted, but they keep showing up without help, and they're not much good until they've grown up and been groomed a bit.

... I have a vague recollection of an epigram along these lines.  It has a distinctly Heinleinian tone, but I can't find anything like it attributed to him.