And that is why a dog can go to the vet and have a really big operation and have metal pins sticking out of its leg but if it sees a cat it forgets that it has pins sticking out of its leg and chases after the cat. But when a person has an operation it has a picture in its head of the hurt carrying on for months and months. And it has a picture of all the stitches in its leg and the broken bone and the pins and even if it sees a bus it has to catch it doesn't run because it has a picture in its head of the bones crunching together and the stitches breaking and even more pain.