Question

Why do rotten egg float ?

Answer 1

Old eggs float in fresh cold water because of a large air cell that forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger and this acts as a buoyancy aid.
Generally, fresh eggs will lie on the bottom of the bowl of water.
Eggs that tilt so that the large end is up are older, and eggs that float are rotten. The tilting is caused by air pockets in the eggs that increase in size over time as fluid evaporates through the porous shell and oxygen and gases filter in. The older an egg gets the more gas builds up inside it. More gas = more floating!