Stereochemistry

Some molecules come in left- and right-handed versions that are mirror images but can't be superimposed, like a pair of hands. Turn 3D molecules with your finger to try to make them match, find the stereocentres that cause handedness, rank groups by the Cahn–Ingold–Prelog rules and name each centre R or S. Then meet molecules with two centres, cis–trans isomers (one of them is how you see), the way chiral molecules twist light, and why the two hands of a drug can act completely differently in the body.