(break into something) to start doing somethingbreak into a run/trot/gallop etc: The children saw the sea and broke into a run.
break into a smile: We watched her puzzled face break gently into a smile.
break into laughter/applause/song: I almost expected him to break into song.
break into a sweat: My legs were shaking with exhaustion, but Phil hadn’t even broken into a sweat.