How do you know if you have rotten egg?

My family and friends are convinced I am an egg expert just because I sell the carton the egg goes into. So they are constantly asking me these random questions. Like... "How do I know if I have a rotten egg?"

This one I happen to know the answer to...

The items you will need to conduct the experiment will be a bowl of water and an egg.

Drop the egg in the water, if it floats it rotten, if it sinks fry it up and eat it or stick it back in the egg carton. 

Its as simple as that

I guess I really am an egg expert ;).