Of course, respond_to? doesn't tell you about what the method does. But
it isn't meant to. To check an object's signature is not the same thing
as to check that it does what you want it to do. A file object that
responds to write() may output a file in the current working directory;
but a mock object which is _acting like_ a file object may just write
to screen, or may not write anything at all. But at least you can tell
that it _acts like_ a file object so far as its signature is concerned
(viz., duck typing).

Regards,
Jordan