Say you had Hydrochloric Acid. This is a very strong acid that can burn you. If you didn't know the pH that said it was an acid (1-7), then it could really hurt you.
Or if you had 3 cups either one of them had bleach, water, and a strong acid. You shouldn't pick one, becuase you could die. Bleach is a base, it has a pH of about 12. Water is neutral and has a pH of 7. But the strong acid could have a pH of 3.
