It may be Summer IRL, but the days are getting colder in the world of Takagi-san, such that puddles made by rain a day ago are frozen solid the next morning. Takagi can tell Nishikata has an unbearable urge to pick that ice up, and so makes a contest out of it.
Naturally, despite Nishikata’s extensive activity in such training, she gets him to drop it by declaring whoever breaks their ice first loses, and has to warm the hands of the winner. He’s about to take her hand when she asks for the hand warmer he offered earlier. A reprieve!
Another morning, Nishikata believes he has a surefire way to get Takagi to lose, by telling her he changed something about his appearance and challenging her to correctly identify that change. This backfires spectacularly when Takagi gives him a thorough visual examination, even tickling his side and getting very close up to smell him.
In the end, Takagi correctly concludes he trimmed his hair, though whether she was going with the answer with the best odds is a mystery. Nishikata’s punishment is to try to guess how she’ll change her appearance tomorrow, knowing full well he’d never look her over as thoroughly as she just did.
After an in-between in which Mina laughs at Yukari’s bangs which she trimmed too far, Valentine’s Day arrives, and Nishikata, while telling himself he doesn’t expect any chocolate from Takagi or anyone else, is still expecting chocolate.
As Mina learns the hard way, Tanabe-sensei doesn’t tolerate the distribution of chocolate on his watch. Mano almost works up the courage to hand Nakai some non-obligatory chocolate, but his ever-present friends ruin her timing and she runs off.
Nakai may be dense but has the good sense to chase after Mano, who is able to successfully give him the chocolate (which he tells Tanabe-sensei is a book she borrowed). As for Nishikata, he goes the whole day without being given any chocolate, only to find a box in his shoe locker, obviously left there by Takagi, who tries to get him to identify her as the one who put it there.
Nishikata doesn’t give in, so Takagi keeps the mind games going by offering him a small piece of obligatory chocolate, while strongly implying that not all the chocolate she gave him is obligatory. Her status as giver of the shoe locker chocolate remains unsaid, but like many things between these two, it doesn’t really have to be said.
There’s no one else from which Nishikata would get non-obligatory chocolate, and there’s no one else to whom Takagi would give chocolate.