A force of habit
Force of habit.
It's a habit.
You better not make a habit out of this.
Habit
A bad Habit.
A magician makes rabbits appear in hats, while a psychologist makes habits appear in rats.
White? No particular reason, it's just a habit they have.
It takes 21 days to make a habit.
A psychologist pulls habits out of rats!
They both made habits fashionable.
Nun
Cold turkey.
He didn't like nuns.