How do people do that face morph effect in photoshop?

There is this one effect that people use in photoshop that morphs the face to take on a different shape but keep the skin color. Examples: 1. http://www.judiciaryreport.com/images/hillary-obama-blend.jpg 2. http://ryanbarrett.typepad.com/.a/6a00d8345259a469e2010536f67264970c-500wi 3. http://www.moolf.com/images/stories/Amazing/Creative-Photoshop-art-by-Pierre-Beteille/Creative-Photoshop-art-by-Pierre-Beteille-7.jpg Disclaimer:I did not make these pictures and i am not trying in any way trying to take credit. Obviousely.