Mix yolk, salt, sugar and pepper together until well blended and yolk lightens in color.
Mix in the vinegar, lemon juice and water (Dijon mustard if using)
While whisking constantly, drizzle in the oil very slowly. Begin with just a drop or two at a time. Once you have added about half the oil, you can add it in a very thin stream. Keep whisking the entire time you are adding the oil.
When finished, the mayonnaise will not be as thick as store bought mayonnaise. Let it sit, refrigerated, for at least half an hour.
Stir, and taste for seasoning. If necessary, whisk in a pinch more salt, sugar or another teaspoon or so of vinegar. Wait before tasting because it takes awhile for the salt to dissolve into all of the oil.