The Doctor: Wibbly-wobbly, timey-wimey. Do you know what the crack is?
Amelia: What?
The Doctor: It's a crack. And I'll tell you something funny. If you knock this wall down the crack would stay put 'cause the crack isn't in the wall.
Amelia: Where is it then?
The Doctor: Everywhere. And everything. It's a split in the skin of the world. Two parts of space and time that should never have touched.