Be careful when working with Nitric Acid that you do not spill any on your skin. It reacts with an amino acid in your skin to form a yellow dye that's much the same color as a yellow highlighter. My college chemistry professor called this "dinitroskin." Your skin may get hard where this happens, but fortunately it only gets the top layer of your skin. Nitric acid will burn you if you don't rinse it off right away (but the yellow compound will have already formed by this time).