its hollow?

It means it's raw, or new, an old egg and a cooked egg loses the air sack that make it bouyant.

An egg can float in water when its air cell has enlarged sufficiently to keep it buoyant. This means the egg is old, but it may be perfectly safe to use. Crack the egg into a bowl and examine it for an off-odor or unusable appearance before deciding to use or discard it. A spoiled egg will have an unpleasant odor when you break open the shell, either when raw or cooked. Safe Cooking Methods Many cooking methods can be used to cook eggs safely including poaching, hard cooking, scrambling, frying and baking. However, eggs must be cooked thoroughly until yolks are firm. Scrambled eggs should not be runny. Casseroles and other dishes containing eggs should be cooked to 160 °F. Use a food thermometer to be sure. Use Safe Egg Recipes Egg mixtures are safe if they reach 160 °F, so homemade ice cream and eggnog can be made safely from a cooked egg-milk mixture. Heat it gently and use a food thermometer. * Dry meringue shells are safe. So are divinity candy and 7-minute frosting, made by combining hot sugar syrup with beaten egg whites. Avoid icing recipes using uncooked eggs or egg whites. * Meringue-topped pies should be safe if baked at 350 °F for about 15 minutes. Chiffon pies and fruit whips made with raw, beaten egg whites cannot be guaranteed to be safe. Instead, substitute pasteurized dried egg whites, whipped cream, or a whipped topping. * To make a recipe safe that specifies using eggs that aren't cooked, heat the eggs in a liquid from the recipe over low heat, stirring constantly, until the mixture reaches 160 °F. Then combine it with the other ingredients and complete the recipe. * To determine doneness in egg dishes such as quiche and casseroles, the center of the mixture should reach 160 °F when measured with a food thermometer.