John grabbed the octopus vial. He had in mind a few things he wanted to do with some tentacles. Rather than drag himself back to Mary across the pool deck, he decieded to take the vial now; octopus arms would be easier to move out of water than a dolphin tail.
John drank half the octopus vial and placed it back in the box of vials. He though he wouldn't need to wait long to turn from a mer-dolphin into a mer-octopus. Indeed, John began to feel himself change. But since he had not also drank half of the human vial, he was not going to get the change he intended. He didn't know this, but as the change began he quickly found out the hard way.