Asking someone to breathe back in the air they have just exhaled seems, on the face of it, to be pointless at best. But doing just that for a very short time will make the person breathe harder. Then when you give them pure oxygen, they will breathe it in really deeply and this might make the preoxygenation quicker. That's the theory. To test it we plan to ask 40 healthy people to do the standard preoxygenation, then try our new way. We won't be giving anaesthetic, just the oxygen.