My father-in-law doesn't like cleanliness very much. I always like to wash my hands in the pool after supper. Sometimes I have a bowl in it and he doesn't care. I will wash it after dinner today.

What should we do? You can have fun with my father-in-law and say that you are a young man and love cleanliness. (Let my father-in-law understand) But you still understand my father-in-law. After all, my father-in-law was born in that era (indicating that you understand his father-in-law), but I still can't accept his behavior, because my father-in-law and I are not from the same era (indicating that I hope his father-in-law understands). That's OK. Pay attention to your tone and your father-in-law's courtesy and sincerity