How does an egg yolk make oil and water mix in mayonnaise?

Mayonnaise is oil and water that have been persuaded to stay mixed. Normally the two separate, but egg yolk contains lecithin and proteins, molecules with one end that likes water and another that likes fat. When you whisk oil slowly into a yolk, the oil shatters into tiny droplets and these molecules coat each one, lowering the tension at the oil-water boundary so the droplets stop merging. Chemists call the result an emulsion: here, oil droplets floating in the yolk's water.

Compare a vinaigrette. Shake oil and vinegar and they look mixed for a moment, then split again, because ordinary emulsions are unstable and need energy just to form. Without a coating, droplets bump into each other and fuse into bigger ones until the oil is back in one layer. Mustard helps a vinaigrette or a mayonnaise for the same reason egg does: it contains a little lecithin.

The packed droplets explain the rest. Mayonnaise is thick because the droplets attract and repel each other at once, holding the sauce like a soft gel, and it is opaque white because the countless oil-water boundaries scatter light.

The trick of adding oil slowly is old. In 1806 the French cook André Viard bound a cold sauce with egg yolk and noted that it held better when the oil went in little by little. Where the name comes from is murkier: most stories point to Mahón in Menorca, taken by the French in 1756, but the word only appears decades later.