How long to cook hard-boiled egg?

You know, I am not a person who is into cooking. Guess I don't ask much about the food I eat either. But then, when you are in the mood of making sandwiches and egg sandwiches to be exact, you will need to know how much time you need to make the egg hard-boiled.

So, to get the answer and to do it correctly, I asked around. Clue number one:
Father said:
If the eggs can turn round and round then the eggs are cooked.

(Tested it out, it is true. Raw eggs cannot go turning round and round because the egg yolk will move around and makes the balancing points vary from time to time when it is turning)

Friend's mum said:
If you can hold the eggs up which chopstick then it is cooked.

(True as well, I don't know the logic)

Nite said:
Boiled it under boiling water for 5 mins.

(Hmmm... I think that is for soft boiled egg?)

Cousin said:
If the eggs are cooked it will sink to the bottom of the pot and it is not, it will float.

(Hahahahaa... one of these days... I have to tell her, that both raw and cooked eggs sink to the bottom of the pot)

Of course I got my egg hard boiled. I boiled in under boiling water for 15 mins and wah la..!!!! It turns out great.... hehehee