Not sure why these do it for me, think it's a combination of innocent looking and 'whoops'! attitude! These aren't the best examples I've seen, but will add to it, please feel free to do the same.
Just to be clear, this isn't 'finger in mouth tasting cum', it's 'finger in mouth looking cute and innocent'!
** Moderator edit ***
Removed attach-tags, no need to use them.
>Ronaldbr<