One day a son asks his father:
"Daddy, will you run the marathon with me?"
The father answers yes and both run their first marathon together.
One day, the son asks his father if he wants to run the marathon with him again and the father answers yes. They both run the marathon together again.
Then one day the son asks his father:
"Daddy, will you run the Ironman with me?" (the Ironman is the toughest...it requires a 4km swim, 180km biking and 42km running).
The father says yes again.
This all sounds easy...but check this video.