Archimedes Principle of Buoyancy says that the upward, buoyant force on an object in a liquid is equal to the weight of the liquid that is displaced by the object. Objects less dense (lighter) than water will float. Objects denser (heavier) than water will sink. The density of most eggs is similar to that of water, but can be a little more or a little less. The exact density varies depending on several factors, including its age an older egg may contain gas that will make it float. An egg that is only slightly heavier than water will sink in pure water but might float in salt water, which is denser because it contains dissolved minerals. There are a lot of answered questions about buoyancy at the SEED web site.