Is that for us?

Today I was erasing the board after one class left and before the next class arrived. A student from the next class walked in as I was erasing and asked, "What's was that on the board?"

I didn't answer, thinking of what I could say. She continued, asking, "What was it? Was it for us?"

I turned around and said, as nicely as I could, "If it was for you, why would I have erased it?"

She says, bright and happy, "Well, maybe you wanted to write it again."