Thanks Dug. I didn't know there was a name for those.
And I agree - they can be very hard to read sometimes.
According to wiki it's
Completely Automated Public Turing test to tell Computers and Humans Apart
Which seems a tad contrived for me.
If they are going to pick the initial letters they should at least be consistent and call it CAPTTTTCHA.
I suppose they could have gone for Public Input Test That's B****y Unreadable Lol Lol
There again that's got acronyms in .... hmm not so easy this.