Per salire le scale SI USANO le gambe (because 'gambe' is plural)

-- Che cosa si usa per salire le scale? -- Per salire le scale si usano le gambe. / Si usano le gambe per salire le scale. => I am not quite sure how to answer questions that start with "si". In Italian, "si" is simply the reflexive pronoun for third-person singular and plural cases. The reflexive pronoun is generally put before the main conjugated verb, as seen in your original question. For a simple affirmative sentence, Italian prefers to have an inversion between the subject and the conjugated verb, i.e., instead of expressing "Le gambe si usano.", we would say "Si usano le gambe." Good day. / Buona giornata. / 祝　是日安好。