Butterfly:
The butterfly stroke is hard on the muscles and also makes breathing difficult. The real problem lies in the coordination of movements in order to take a breath at the right time.
Air is taken in through the mouth, at the end of the pull-push phase, by lifting the head so as to look at the surface of the water. It must be fast so that the head immediately returns to its original place in order to keep the stroke as balanced as possible. For better "hydrodynamics", a breath is usually taken every other stroke. Exhaling however is continuous through the nose and mouth once the head is in the water.