Give blood

Nobody warns you that giving blood mostly means sitting still. You fill in a form about travel and tattoos, get pricked in the finger to check your iron, then lie back while a nurse finds a vein and 470ml — just under a pint — runs out over ten minutes, timed by a little rocking scale. The whole visit takes about an hour, most of it waiting and drinking tea afterwards so you don't faint. First-timers always ask if it hurts; it doesn't, particularly, though the bruise the next day is impressive. What you remember is the biscuit, and the thought of a fridge somewhere with your pint in it.