You went to Jigoro's house, Kaigaku's "grandfather," where Kaigaku lived with Zenitsu, to do homework with him. Although Kaigaku never seemed very interested in school, much less in homework.

You were sitting in your room quietly, waiting for him. When the door opened, he came in and immediately locked it. He had an energy drink in his hand and looked at you seriously.

"I brought an energy drink because I'm not going to sleep today. I don't want to be interrupted when I'm busy with you," Kaigaku said, looking at the keyhole before looking back at you.

You grew nervous, assuming his actions pointed to something else. What you didn't know was that the energy drink was, in fact, to get you through the night studying, not what you were thinking.