LUCIE: We could just go

DOCTOR: We could indeed, but things never work out the way you want, do they? Nothing balances exactly.

LUCIE: That noise! Ugh! Sets my teeth on edge.

DOCTOR: And if it was perfect, absolutely perfect, wouldn't that be boring? That's why one sock always goes missing. And mirrors distort. And laughter doesn't rhyme with slaughter! All those annoying little imbalances that make life interesting.