I just wonder, if someone wanted to spam the site, they could easily create rhyming words by just adding a few letters to the last letters of the required word, for example:

rhymes with: note?

would snote or rnote pass the test? what about denote? how about Orange?

ok, maybe not the last one, but really how precise could the check be to still allow humans to pass the test?

Well, it'd have to be English. Not only does this system require the ability to rhyme but the words have to be spelled correctly.

Already an improvement!

The note/de-note issue is a possible vulnerability. I'm not sure by how much. We'd probably need a work-around.