funny funny, where does this come from?

Sarah and I were talking and Maddison came up to us and said Hey Sarah, come with me. Sarah said no I'm talking to mom right now. Maddison stood there for maybe 10 seconds and then asked the same question again. I said Maddison, Sarah already answered that question and the answer was no. Maddison turns around and says to herself, "oh Sarah said no, its my fault." We started laughing so hard. I don't even know where she learned what fault means, I haven't ever really said that word. It is so funny the things she picks up without me even trying!