In just three short lines, the poem tells the story of a child whose natural curiosity is shut down by the very people meant to nurture it. The title, "Shut Up," is blunt and harsh, and it sets up the command the parents seem to give again and again. The child asks questions, wants to know things, and is told to stop.
The image of parents who "zipped his mouth" is the heart of the poem. A zipper is quick, mechanical, and easy to close, which suggests how casually the silencing happens. It also feels a little violent, as if the child's voice is something to be fastened away rather than listened to. The word "Yet" marks the turn: curiosity should have been welcomed, but it wasn't.
The speaker never states a feeling outright, but the flat, restrained tone suggests sadness and quiet criticism. The mood is cold and sobering, and the poem's brevity mirrors the cutting-off it describes.