Sandpaper is a smoothening tool, it used to smoothen rough edges. I actually thought sandpaper was just something people say to refer to something smooth, I never knew it was a tool that really exist until I grew. There use to be a quote on sandpaper, so I assumed they were referring to the quote and not the real stuff. Imagine my surprise when I discovered the truth.
A layman might say sandpaper is a paper with sand used to smoothen rough edges.
The term, "smooth as sandpaper" is an expression that gives the smoothening ability to something else. The figure of speech used here is simile. But in reality, there are lots of things as smooth as sandpaper, some even smoother.
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