Why isn't an older mitten a mat?

From Donna WilsonI came across this charming rhyme posted as a comment on a Facebook thread, and can't resist copying it to my blog.

The Kitten sat on the Mitten

And the young of a cat is always a kitten,
but the young of a rat is never a ritten
and conversely a kitten when older's a cat
so why isn't an older mitten a mat?

Michele Brenton, aka Banana the Poet, has published books of her witty and accessible poetry, which you can find (and buy for a modest price)  here .