Carefully put a fresh egg in a glass of plain tap water. It will sink to the bottom of the glass. Fill another glass with warm water and add kosher salt, pickling salt or sugar. Table salt may be used, but it will make the water murky because of the additives in the salt. Add the egg to the glass of salt or sugar water. It should float. If not, keep adding salt or sugar to the water until the egg floats. The egg sinks in plain tap water because its density is greater than that of water. However, salt water is denser than both plain water and the egg. Therefore, the egg floats in salt water.
Add 4 tablespoons of salt to 2 cups of water. Stir together. Half-fill a glass with the salt water solution. Slowly add tap water to the salt water without disturbing the layer of the salt water. Carefully drop a fresh egg into the glass. It will sink through the plain water and come to rest on top of the salt water. Because salt water is denser than plain water, plain water will float on top of the salt water. The egg is denser than plain water, but less dense than salt water. The egg stays suspended in the middle of the glass.
Fill the bottom fifth of a glass with salt. Add just enough water to the glass to make a wet layer of salt. Carefully lower a fresh egg on top of the salt layer. Slowly finish filling the glass with water without disturbing the salt layer. Cover the glass with plastic wrap and secure it to the top of the jar with a rubber band. Observe the egg for weeks. As the salt dissolves, the plain water will become saltier and the egg will slowly rise. As the egg rises, mark its progress with a felt tip pen on the outside of the jar.
Age an egg for weeks or months. Set it aside in the refrigerator so that it is not confused with fresh eggs. Carefully drop a fresh egg in one glass of water and the old egg in another glass of water. The fresh egg will sink while the old egg should float. This is due to a small pocket of air inside the egg. As an egg ages, it loses water and carbon dioxide through its shell. The size of the egg's air space increases. The egg's density is less than when it was fresh, but its volume stays the same. The increased air space also makes the egg more buoyant. The old egg becomes less dense than the water and floats.