to which my answer is always, even though I'm not a doctor, 'Well, don't do that!'.
An old gag. I don't remember the name of the film anymore but I remember seeing an old British movie in the 70's where some patient was asking for a doctors certificate. 'Doctor', he said, 'after the accident I can only raise my arm this high'. 'Oh', says the doctor, 'how high could you raise it before the accident?'. 'This high' says the patient, raising his arm much higher. You saw that coming but I remember laughing until it hurt at the gag.
That's become one of the family gags with my US family. Andrew in particular seems to relish it. It all came about because of Corey, a kid who lived across the way from us. Nice kid. One day he came over with a suction cup. A bit of spit and he sticks it on his chest, where, naturally enough, it sticks. Then he did something or other to make it pop and at the moment it popped 'Ow!'. Did it again. 'Ow!'. And again. After maybe a dozen ows he had quite the red mark on his chest. Nothing daunted he moved the suction cup to the other side of his chest and repeated the performance. I could barely restrain my laughter.
All of which reminds me of another story. I wish I could say this was about Robin because it would fit very neatly into the 'you bastard' series. Alas it isn't and it doesn't.
One of my friends back in 1968 was Mark. Nice kid but not the brightest penny in the purse. I've learned in the 38 years since that one doesn't take advantage of such folk but back in 1968 that lesson was still on my 'to learn' list.
So one day I taught him a trick. You take your fist and hit, very very hard, just below the elbow. Maybe an inch or so below, having taken the precaution of laying your arm on the table. If you do it just right the muscle bulges for a moment and then subsides. Pure bulldust but he believed. So he tried it. 'Did you see it?' he asked. I had to admit I hadn't. He tried again. 'Did you see it?'. 'Nope!'.
So he tried and tried, for what seems like, now, half an hour, though I'm sure he gave up after five minutes or so. What I do remember is that when we left the classroom that day I was laughing myself sick and his arm was swollen up and red and sore.
Doctor, it hurts when I do this! Well don't do that!!!