I'd ask any person who doubts that 0 is even: If I subtract one even number from another, is the result always even? If they say yes, then they just proved th

I'd ask any person who doubts that 0 is even: If I subtract one even number from another, is the result always even? If they say yes, then they just proved the even-ness of 0. If they say no, then they must allow for some exception. Like, the result is even only for positive results. Then I would ask them what purpose is served by having that exception.