Mayonnaise is a smooth sauce made from oil and the yolk of eggs. It is an emulsion in which the oil is dispersed in tiny droplets. Egg yolk contains lecithin which prevents the oil and water from separating out. Mustard is also a good emulsifier and is frequently added. Vinegar, usually wine vinegar, or lemon juice are added for flavour and because acidity is necessary for the emulsion to properly form. Garlic is a frequent addition. The oil used is normally a flavourless type such as sunflower oil or corn oil, but you can substitute some for a stronger tasting oil such as olive oil.