My son says that the way you can tell

A male from a female cat,

Is, if you scratch its back,

And it just lays there, it's a boy.

But a girl cat can't help but arch her back 

And get up and slink into you,

And get you to rub her back some more.

And I don't know if this is true,

But this is what my son thinks,

And that, in itself, is very interesting to me.

-jenn