You are participating in a marathon race. It is a hot and sunny day. There are still 10 km to run, but you feel totally wrecked; the soles of your feet hurt and burn, you feel pain in your knees and hips, your thigh muscles are cramped, and your dizziness is increasing exponentially. Your mind keeps drifting away. Suddenly you stumble on another runner's foot and fall helplessly flat on your face. In your fall another runner falls over you. It turns out to be wiry old woman, seemingly around 80. She immediately gets to her feet and hiss at you in rage. Amateurs like you shouldn't be allowed here! A couple of hours later, when you sit on the grass in the finish area, eating a banana and drinking from your water bottle, you hear an ice-cold voice behind you. Look what you did to me! You turn around - and there she is again, that wiry old woman, pointing at her bloody knee. You get to your feet with difficulty, trying to figure out what to say.