In this glorious hadith, after having his clothes urinated on by his grandson, Muhammad teaches that girl's urine is worse than boy's urine.
So why is girl's urine worse?
Ibn Uthaymin writes:
If it is said, “What is the reason why the urine of a boy who is not eating food is sprinkled and not washed like the urine of a girl?” The answer is that the reason is that this is what is prescribed in the Sunnah, and that is sufficient reason. (Sharh al-Mumti 1/437-38)
While this standard “We must blindly follow Muhammad” explanation doesn’t cut it for me, the explanation given by Imam Shafii—of the Shafii madhhab—is more than I could ever dream for:
Narrated Abul-Yaman al-Misri:
I asked Shafi'i about the Hadith of the Prophet, "Water should be sprinkled over the urine of a baby boy, and the urine of a baby girl should be washed," when the two types of water (urine) are the same. He said, "This is because the urine of the boy is of water and clay, but the urine of the girl is of flesh and blood." Then he said to me: "Did you understand?" I said: "No."
He said: "When Allah the Most High created Adam, He created Eve from his short rib, so the boy's urine is from water and clay, and the girl's urine is from flesh and blood." Then he said to me: "Did you understand?" I said: "Yes." He said: "May Allah cause you benefit from this." (Ibn Majah 525)