The letters r and n, when written together in lower case (rn) can easily be misread as the letter m. The blanks in both expressions below can be filled in with the same letters, in the same order, to make two common phrases. What are they?

_ _ rn   _ _ _
_ _ m   _ _ _

HINT:
One of the phrases means to reach a point where you can't go on.

Answers:

burn out
bum out