Cats are mind readers. It is very important not to even think about cat boxes when you need to take someone to the vet, because they know instantly!
Of course, my girls read my mind when I think about even going to the vet.

But...when I'm at the shelter and a cat has been adopted, almost always they go right into that carrier on their own. Take me home they say! Opening that carrier door for an adopted cat has become my favorite part of volunteering.