No follow-up riddle, so I'll enter one:

Mark and Billy have been the best friends ever since they were little kids. They are also very competitive. Throughout the years they have challenged each other to do both physical and mental challenges. And they completed the challenge. But one day Mark thought of something to challenge Billy to do - something he could start but never finish. The average man could do it and so could Mark and they were both the same sex and the same size. It is a physical challenge. Can you figure out what it was?