第1話: 楽器の掃除について説明する久美子に、友人たちは醤油やケチャップ、塩を提案する

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One day, Kumiko reveals that she brought her instrument home only to give it a good polishing. She says it takes time to clean the instrument on the inside, despite it looking shiny on the outside, and that you can clean it by putting it in a bathtub. Hazuki misinterprets this as taking a bath with your instrument. Midori says that cleaning your instrument with soy sauce works, but Kumiko says that it's ketchup that works. Reina, however, prefers salt. (Source: Hibike! Euphonium Wiki)