Two astronauts were in a space ship circling high above the earth.
One had to go on a space walk while the other stayed inside.
When the space walker tried to get back inside the space ship, he discovered that the cabin door was locked, so he knocked.
There was no answer.
He knocked again, louder this time.
There was still no answer.
Finally he hammered at the door as hard as he could and heard a voice from inside the space ship saying, 'Who's there?'