Complicated things can be explained by examining their individual parts. Complex ones cannot. They are always greater than the sum of their parts. This dynamic has nothing to do with the number of parts or, say, the cost of the object. A jet engine is complicated. Mayonnaise is complex. You can easily replace a part in a jet engine and not alter its fundamental nature. It’s still a jet engine, though possibly inorperable. With mayonnaise, if you change one ingredient, you run the risk of altering the essence of its mayonnaise-ness. What matters is not the components alone but how they interact with one another. (Speaking of interactions, people tend to describe themselves as complex and their spouse as complicated).

from The Geography of Genius by Eric Weiner

Recipe and image from http://www.epicurious.com/recipes/food/views/mayonnaise-241083

Whisk together yolk, mustard, and 1/4 teaspoon salt until combined well. Add about 1/4 cup oil drop by drop, whisking constantly until mixture begins to thicken. Whisk in vinegar and lemon juice, then add remaining 1/2 cup oil in a very slow, thin stream, whisking constantly until well blended. If at any time it appears that oil is not being incorporated, stop adding oil and whisk mixture vigorously until smooth, then continue adding oil. Whisk in salt to taste and white pepper. Chill, surface covered with plastic wrap, until ready to use.