Here are your needed supplies! 1 egg, 1 cup.
Step 1 : Fill the cup with around 3/4 water.
Last step : Place the egg in the water and wait a couple of seconds. If the egg floats up, the egg is rotten. If the egg sinks down on the surface level of the cup, it is still safe to eat!
Rotten eggs float up because the natural air cell in the egg becomes larger. The more the egg ages, the more air gets in the egg and acts as a lifting agent, making it not safe to eat. Enjoy!