You say sh when you want someone to stop talking or to make less noise. It is not a full sentence. It is the quieting sound written down: lips almost closed, air pushed out. People often put a finger on their lips at the same time. Parents say it to children; ushers say it in cinemas; strangers say it on a train when a phone is loud. You can stretch it — Shhhh — if a short sh is not enough. It feels informal and a little sharp. A teacher may say Quiet, please; at home you will hear Sh! though actually that can sound annoyed, as if the noise has already gone on too long. Softer cousins are a longer shh, or hush. It is the whole word, not the sh- sound inside she or ship.