Why can U hear a teacher through the closed door of a classroom?

You can hear a teacher through a closed door because sound waves can travel through solid materials, albeit at a reduced volume. When the teacher speaks, the sound waves created by their voice vibrate the air, and some of these vibrations can penetrate the door. The door, though it blocks some sound, doesn't completely seal it off, allowing some of the sound to pass through and be heard outside. Additionally, sound can also travel through cracks and gaps around the door.