On Wed, 30 Oct 2013 21:16:42 -0700 (PDT), Helpful person
> wrote:

>On Wednesday, October 30, 2013 3:38:36 PM UTC-4, Brooklyn1 wrote:
>>
>> Flatness doesn't matter for even heating with a gas stove. Most
>> quality cookware is designed with a slightly concave bottom that will
>> become flat when metal expands as it's heated.
>
>Why will it flatten when heated?

Metal expands when heated. It will sort of push the sides out and
flatten. Some pans are designed that way to allow for the expansion.
A metallurgist can explain it better.