K-ON!! Season 2 episode #5
PlotAzusa and Ui, along with their classmate Jun, spend time together while the third years are on their trip to Kyoto. Noticing that Ui would be lonely without her sister around, Jun and Azusa decide to sleepover at her house, in exchange for Azusa helping out with her underclassmen in the jazz club.
After spending the night, they wake up to a rainy day. Their plan to go to the zoo was spoiled. They decide to go to an arcade where Ui hits a home run in a batting game, before Azusa is reminded that she needs to feed Ton-chan. While in the music room, the three decide to have a jam session together. The other girls return the next day and give Azusa a keychain (bu), one of a set of five that spells out 'light music club' (keionbu).