An egg can float on:

Because the mass of the salt water displaced and the mass of the egg are equal, an egg floats in salt water. The density of the egg is lower than that of salt water. An egg rapidly sinks to the bottom of the container it is placed in when it is submerged in fresh water. Because salt washes off of rocks and into the ocean, where it has accumulated through time, the ocean includes salt water. When the egg is placed in the glass after salt water and fresh water have been carefully layered on top of one another, the egg falls through the fresh water but is supported by the salt water. The egg floats between two layers of water because it passes through the fresh water but is held up by the saltier, denser water. Due to its lower density, fresh water will float on top of salt water.