I don't understand why people have trouble with this. Perhaps because they are so used to zero being an exception to things? Perhaps because the sheer act of

I don't understand why people have trouble with this. Perhaps because they are so used to zero being an exception to things? Perhaps because the sheer act of raising the question makes them nervous? The everyday definition of even -- divisible by two -- applies just fine to zero, nor are there properties of evenness or oddness that should make one doubt whether the definition is appropriate. Questioning whether zero should be even is like questioning whether six should be even. It meets the definition, and there are no issues, so what motivates the concern? Of course, one could always make another definition, but what you'd really be doing at that point is appealing to the mathematicians around you that you have a better definition of even. To them, that means one that makes things simpler. And it's unlikely that you have one. I once had the following conversation with my abstract algebra teacher, while in grad school: Him: If you study the history of this particular field, as time goes on, the definitions get more complex, while the theorems and proofs get simpler. Me: Why is that, do you suppose? Him: It's progress!