Always
Assume it's
Acid. A good
law to go by in a
lab that has
wet benches, and lots of
unlabeled beakers filled with
innocent-looking clear liquids. You do not drink the liquid or let it splash on you or mix with other liquids, because you
assume that it might be acid.
Also,
Always Add Acid. Meaning, when you're making a diluted acid solution out of acid and water, put the water in the container first, then add the acid to the water. Putting a small amount of water into a large amount of acid makes all the acid want to jump into the water and mix with it at once, creating an exothermic reaction, and possibly big trouble.