Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
Carefully place the egg into the drinking glass and fill with water until the egg is fully covered and the glass is about 3/4 full. Observe what happens to the egg: is it standing up? lying sideways?
Add a few large spoonfuls of salt into the glass and stir until all of it dissolves. If you like, you can take the egg out of the glass to make it easier to stir.
Continue adding and dissolving salt into the water until the egg begins to float. At this point, the saltwater solution we've made is actually more dense than the egg. And so, like oil floating on top of water, the egg floats too!