You could also clean your pennies with salt and lemon juice or orange juice, because those juices are acids, too. To understand how the nail and screw got coated with copper, you need to understand a little bit more about atoms. Atoms are made up of even smaller particles called protons, neutrons, and electrons. Electrons and protons are both electrically charged particles. Electrons are negatively charged and protons are positively charged.
Negative charges attract positive charges, so electrons attract protons.
When you put your dirty pennies in the vinegar and salt, the copper oxide and some of the copper dissolve in the water. That means some copper atoms leave the penny and start floating around in the liquid. But when these copper atoms leave the penny, they leave some of their electrons behind. Rather than having whole copper atoms in the liquid, you've got copper ions, copper atoms that are missing two electrons. These ions are positively charged.