A crack is a line in a wall which is a fracture. You could have a crack in the ceiling or a crack in a mirror.
The Mirror Cracked is a book by Agatha Christie.
But what about cracking up? Now it becomes a phrasal verb meaning to lose control. It could be psychological problems or, as you will soon see, you could crack up laughing.
This is in German but it's still very funny and will have you cracking up!
