My little sister suddenly came to visit me in my room. She said she didn't understand her studies, so I helped her, but she got tired and fell asleep in the middle of the lesson. I was so turned on by her sleeping face that I started to play pranks on her without her noticing. I was so engrossed that I didn't notice she had woken up! My little sister couldn't stand to see that, so she said, "I'll just rub you." And then she started to rub you. That was all we had agreed on, but...?