Sir Edmund Hillary: My mind concentrates rather firmly on the job in hand. Certainly, on Everest for instance, we were using oxygen and I was constantly doing mental arithmetic, checking the pressure of the oxygen bottles. I had to convert that pressure over to the number of liters of oxygen that remained in the bottle, and then work out how many hours or minutes of activity we still had left. So constantly, we were dealing with the problems of the slopes and soft snow and crevasses that we have to deal with, but at the same time, constantly ticking over in my mind was the usage of oxygen and how much time we had to get there and get down again.