It is standard practice for patients to breathe oxygen before receiving a general anaesthetic. This is called preoxygenation. It is done by using a mask which fits snugly onto the face. But for how long should patients breathe oxygen? Is taking deep breaths a good idea? What is the best oxygen flow rate? These questions are important because preoxygenation is one of the things which makes anaesthesia safe, so finding the best way of doing it might help save lives. Previous research has attempted to identify the best way to give oxygen before anaesthesia. It seems that breathing pure oxygen for three minutes is enough, or if you are in an urgent situation, then taking deep breaths of pure oxygen for a minute will do. However, there is another possible way of doing it which has not yet been looked at.