The longest shared border with the US is...
According to a moron on an American show airing on Global, it's Mexico. I was screaming at the TV that she should say "China", but that's only because she didn't realize that the sun was a star (for that answer, I disagreed with her...my answer was Sol, which is more correct). Yeah...I don't know where they find these morons (BTW, she didn't know how many sides a trapezoid has)