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A tetrahedral carbon attached to four different substituents can give non-superimposable mirror-image enantiomers. Chirality is necessary, but not sufficient, for optical activity: a racemic mixture has equal amounts of both enantiomers whose rotations cancel, while a meso compound is achiral because of internal symmetry. The direction of rotation, marked (+) or (–), does not follow from R/S configuration. Specific rotation normalizes observed rotation for path length and concentration, and measured rotation also depends on conditions such as wavelength and temperature. The lesson connects these ideas to enantiomeric excess, separation of enantiomers, and stereochemical outcomes in reactions.