Cafe Hayek produces an elegant example for use when talking to people who have a problem will illegal immigrants because they broke the law to get here:
For me, an illegal immigrant who comes here to work is like a father speeding to the hospital to get his son medical care. When he arrives, the hospital could say:
I'm sorry, I wish we could take care of your kid, but you broke the law on the way over here. You were speeding. So we can't give you medical care. That would reward criminals--people who break the law by speeding.
I like these kinds of analogies; I like the amount of emotion they (rightfully) inject into the debate.
But what hospital would say that? Everyone speeds on the way to the hospital. Everyone understands that speeding, while always illegal, is only immoral when it endangers. And we pardon speeding under circumstances such as a sick child on the way to the hospital. Why do people want to keep out those who come here to work, legally or illegally? What does the legality have to do with it?