I Don’t Get It

I had my friend over for dinner the other day. The recipe called for three cups of water. I get out the measuring cup,fill it with water and pour it in. When I was done, I dried it off and put it away. My friend said, “aren’t you going to clean it off before you put it away.” I said, “why, I only put water in it. What do you want me to do, put more water in it to clean out the old water that was in it.” She just went on saying that I should clean it out, because it was used.
I don’t know. I don’t get it.