We've been working on getting Stephen to wash ("scrub") himself in the bathtub. He's at least willing now, so that's good. But, the past few times, he's tried to wash his bottom first. We'll stop him and remind him "face first, bottom last." He's getting there.
This reminds me of a similar situation when I must have been about his age. I don't know if my mom remembers this, but I believe I had some confusion along the same lines. The memory of the situation is a bit vague, but not of the punchline. I was in the tub and mom reminded me that we wash our bottom last. I asked why (of course), and she said it's because that's our dirtiest part. Being the logical child, I asked what happens if some other part of my body was dirtier. I clearly remember her response. She looked at me very seriously and said: "then you have a problem."
I vaguely remember that being enough for me at the time. I suppose it still is. But I have the feeling she was trying not to laugh at the time.