BakaHoushi said: Well, that's quite... Out of character for Sakaki.
This is a scene taken from the new strips at the end of the 10th anniversary edition of volume 2.
Osaka gets whacked on the head by one of those lowering guard arms at a railroad crossing, and it makes a lump on her head. She's talking to Sakaki about it after class, and as Sakaki is feeling for the lump on Osaka's head, Osaka ponders what's inside those lumps. This was Sakaki's response.