Mindbenders from Einstein

Back when I was a kid we used to do these mindbender logic problems all the time in one of the classes that I was in. I enjoyed them so much that I spent quite a bit of time over my summers doing them to pass time. This one is extraordinarily difficult. Einstein reportedly said that 98% of people cannot complete it. Can anyone else get it?

Who has the fish?

Note: Coudal has a link to the answer if you want to cheat!