>Ok, then:
>
>

>Tom's and Jerry's cars.
>

>
>is proper?
>
Depends upon context. If it is one set of cars owned jointly by Tom and Jerry, then it would be Tom and Jerry's cars. If it is one set of cars owned by Tom and another set owned by Jerry, then Tom's and Jerry's cars (being an equivalent of Tom's cars and Jerry's cars).


>It's amazing that Americans ever learn to talk...

Talk, yes.

Correctly???? Body language and intonation will often convey information that won't come across from a written statement. Which is why grammar goes out the window in spoken language.