When you break the yolk of an egg and fry it on both sides, it is commonly referred to as a "sunny-side-up" egg. The yolk remains runny and exposed to direct heat while the egg white cooks on the bottom. It's called "sunny-side-up" because the yolk resembles a bright sun on the top of the egg white. If you prefer the yolk fully cooked, you can cover the pan briefly to allow the steam to cook the top part of the egg slightly. This style of cooking eggs is a popular and delicious breakfast option for many people.