These two monks, a newly minted young one, and a wizened older monk, are walking down the road in springtime. They come across an old woman standing next to a river ford. The river is moving at a pretty decent clip, and it's swollen with spring runoff, so it's pretty evident that the old woman isn't going to make it across on her own.
Because the monks belong to an order that believes in doing good deeds, yet totally abhors contact with women (women are the downfall of mankind), the young monk continues past her and crosses the river. The old monk carries the old woman across on his shoulders, and sets her down on the other side of the ford.
The young monk is obviously affected by this, but he continues to fume in silence. After a couple miles pass, the young monk can contain his emotion no longer, and begins to berate the older monk for having contact with the woman. "Why did you soil yourself with contact with that woman. She's unclean, and the cause of the downfall of man..."
The old monk halts his tirade with a gesture and calmly says "I ceased carrying her two miles ago. Why do you still carry her?"