As a society, we have changed our culture to be big consumers of soft foods. This trend appears to have led in turned to progressively more alteration of jaw development and in some cases too little room for the last molars (wisdom teeth) to erupt (emerge from gums), a phenomenon known as “impacting” of wisdom teeth-Daniel Lieberman summarized the results: “The mechanical forces generated by chewing food not only help your jaws grow to the right size and shape, they also help your teeth fit properly within the jaw.” Changes in chewing changed human jaws and faces.-soft foods such as fruit, yogurt, applesauce, and peanut butter do not provide the chewing opportunity that children need. The same lack of opportunity is suffered by children of parents who cut meat and other chewable food into tiny pieces because they are afraid their kids might choke. Therefore, children miss out in the learning experience of how the tongue and muscles coordinate to maximize extraction of nutrients from a meal and so not get the muscle exercise that is needed for the healthy development of the jaws

As you chew less hard foods, your jaw muscles get slack, and thus you hang out mouth open due to an under exercises and weakened jaw muscle.