A few further thoughts on "things impossible," since it's a popular motif. It's the core motif of "The Elfin Knight" [Child 2], aka "The Cambric Shirt" or "Scarborough Fair." In America, there are a lot of texts along the lines of "The Barefoot Boy with Boots On"/"While the Organ Pealed Potatoes."

Lewis Carroll of course used it to great effect in "The Walrus and the Carpenter":

The sun was shining on the sea,
      Shining with all his might:
He did his very best to make
      The billows smooth and bright --
And this was odd, because it was
      The middle of the night.

The Thompson Motif Index has about six pages of entries for "Impossible Tasks," starting at H1010 (compare Tale Type 428); H1020, in particular, is "Tasks contrary to laws of nature." I am not going to scan every entry on those six pages :-), but H1010 alone has citations including the Child collection, the Arabian Nights, the Brothers Grimm, an Irish myth, and things from India, Indonesia, and China. And that doesn't even mention "Culhwch and Olwen"!

And I know that was more than you wanted. :-)