Compare to or compare with?

The engine room is not here to teach English but I really, really wish the writers in our care would get the message that there's a rule governing the preposition that follows the word "compare", and it's this: if you're comparing two things to show that they are similar you say "compare to". If you're comparing two things to show they differ you say "compare with".

The usual reference for this one is Shakespeare's phrase: "Shall I compare thee to a summer's day".

It's easy. But after repeated reminders some of our charges simply can't get their heads round it. Anyway, I do feel better for getting it off my chest – which is what this blog is really about... stress relief for knackered subs!