One egg yolk can hold roughly a cup of oil in suspension if you add it slowly enough, and that ratio — a yolk to about a cup of oil, plus a spoonful of acid and a pinch of salt — is the entire recipe underneath every dressing, dip, and sauce that lists mayonnaise as an ingredient.
Lecithin does the actual work
Egg yolk contains lecithin, a phospholipid built with a water-loving head and a fat-loving tail. Whisking splits the oil into billions of microscopic droplets, and lecithin wraps around each one, water-loving side out, fat-loving side in. That coating is the only thing keeping the droplets from finding each other again and re-forming a separate pool of oil, which is what a broken mayonnaise actually is, mechanically.
Why the first few drops matter more than the rest
Lecithin from one yolk has a fixed amount of surface it can cover. Early on, before much emulsion exists yet, adding oil quickly floods the yolk with far more droplets than it can coat before they touch each other and merge back into oil — the mixture looks like it's about to split because it is. Once a thick base of tiny, stable droplets has formed, that base helps shield anything added after it, so the oil can go in as a thin stream rather than drop by drop.
What broken mayonnaise looks like, and the fix
A broken emulsion turns thin and greasy, and separates into a puddle of oil sitting on a curdled yolk mixture. Nothing subtle about it. The fix is not to keep whisking harder. Start a fresh yolk, or a teaspoon of water, or a spoonful of mustard, in a clean bowl, whisk it alone for a few seconds, then add the broken mixture back in the same slow way you'd add oil. The fresh base supplies lecithin the first batch ran out of — though not every separated batch fixes the same way.
Mustard, temperature, and what makes the yolk's job easier
Mustard is not only flavor here. Its natural mucilage acts as a second emulsifier and makes the mixture more forgiving of a fast pour. Room-temperature ingredients emulsify faster than cold ones, because cold oil is thicker and breaks into droplets less readily, and a cold yolk's lecithin distributes more slowly through the mixture. Neither is required. Both buy you margin for error.
Scaling up, and mayonnaise made with a whole egg
The ratio holds as you scale: two yolks handle roughly two cups of oil, and a batch that's short on emulsion needs more yolk added in that same proportion, not more oil poured onto a fixed amount of yolk. Swapping in a whole egg instead of an extra yolk works too — the added water thins the base and makes the emulsion more forgiving, at the cost of a lighter, less rich result. That's why blender and food processor mayonnaise recipes often call for a whole egg rather than yolks alone.
The one real safety line
Homemade mayonnaise made with raw egg yolk carries the same small Salmonella risk any raw egg does, and nothing about the process cooks it away. Keep it refrigerated, use it within about a week, and reach for pasteurized eggs instead of raw ones if you're serving anyone pregnant, elderly, or immunocompromised.