Dear Doctor Adamson, the linguistics professor. Well, he's not all the dear.
In explaining the difference between allophones and phonemes, he used this analogy:

If we're talking about cars, we can talk about Fords, and Chevrolets. Then, within Chevrolet, there are Corvettes and Cavaliers, but that doesn't make a difference. I only care if it's a Ford or a Chevy.

Come again? It makes no difference if it's a Corvette or a Cavalier? Have you ever driven a Cavalier? If you promised me a Corvette and showed up with a Cavalier, you'd better believe it'd make one hell of a difference. You'd be hearing about it. I'm sure that nice, educated little brain of yours can come up with a new metaphor.