When copper sits exposed to air for any extended period of time, the reactive metal will tend to oxidize and form a layer that tends to interfere with the soldering process. And flux will do some part to clean up that oxide and reduce it and make a clean metal surface. But often times if you have a lot of oxide present, it's helpful to remove it mechanically. And for that we use sandpaper. For example, I have two pieces here, one of which I'll sand. If you don't see shiny metal, it's likely that you need to use sandpaper. And by sanding, you can see that the copper appears much, much different. It's a little bit shinier and the color is different. So now that we've removed the oxide layer with the sandpaper, we have a nice clean metal surface for which the solder can adhere better, we use less flux and we'll get a better solder joint.