This document discusses acids and bases. It defines acids as sour tasting substances that produce hydrogen ions and turn litmus red. Common acids include those found in vinegar, citrus fruits, and other foods. Bases are defined as bitter tasting and soapy substances that turn litmus blue. Common bases include baking soda, detergents, and antacids. The document also explains how litmus paper can be used as a natural indicator to test whether a substance is acidic or basic by changing color.