Cooking an egg "over" refers to frying it on both sides. Whether the yolk is kept runny or cooked to firmness is what is meant by the terms "over easy" and "over hard." A solid cooked egg yolk is cooked to the hard state. The technique for cooking an egg over hard to perfection is to adjust the temperature so it cooks at an even rate. Cooking it too quickly will burn the edges and give it a rubbery texture.