Hikari Kohinata and Futaba Ooki go to the Diving Club room to apply for the tryout period. Hikari barges in with enthusasiam, and Futaba nervously enters in afterwards. When they enter, they learn that the Diving Club advisor is Mato Katori, their homeroom teacher. She asks if they might have any idea who had broken into the Diving Club room the previous day and Hikari immediately confesses that it as her. Mato hits her over the head.
Hikari and Futaba explain that they're there for the tryouts, and ask if they are able to get into the pool during the tryout period. Mato explains that before they're allowed in the pool they'll need to pass some theory lessons. She begins by teaching them about changes in air pressure - the feeling when your ears hurt when riding on a plane because the external air pressure changes, which causes the internal air pressure to push against the airdrum, causing pain.
The same happens in the water - the deeper you dive, the greater the pressure on your whole body, the volume of air in your body decreasing. This is counteracted underwater by breathing in more air than normal to maintain the air volume in your body and clearing the ears by holding your nose, closing your mouth and breathing out, which forces air into your ears. They ask Futaba to try it, and she does so successfully, popping her ears. Mato then explains that the same is true when ascending from a dive; as you ascend, the volume of air in your body will increase. As such it's important to never hold your breath underwater - by breathing in and out, air pressure will naturally equalise.
On understanding these theory lessons, Futaba suddenly realises that diving isn't so intimidating after all, that she's able to understand it like puzzle pieces fitting together, and that she's extremely interested in diving. She realises that Hikari's enthusiasm has rubbed off on her - she's excited to go into the water. Caught up in the enthusiasm, Futaba calls Hikari by her nickname - Pikari - for the first time, which makes Hikari so excited that she tackle-hugs her.