The yolk is the round yellow part in the middle of an egg. The clear part around it turns white when you cook it, and that part is called the white; the yellow ball in the centre is the yolk. In a raw egg the yolk is soft and will break if you touch it, and it runs out thick and orange-yellow. In a hard-boiled egg it is dry and crumbly. People talk about yolks a lot in cooking, because many recipes want only one part of the egg: 'separate the eggs' means take the yolk out and keep the white apart. Cooks say things like a runny yolk (still soft, it runs when you cut it) or an egg yolk (the full name, very common, even though yolk alone already means the same). In biology, the yolk is the food store for the baby bird growing inside the egg — that is what it is really for, and you will meet the word this way in science texts too, sometimes about fish and other animals, not only chickens. A few English speakers also use yolk in a loose way for any yellow centre of something, but that is rare and playful, not standard. The word is a countable noun: one yolk, two yolks. It is not usually a verb.