The best reason I can think of for why we believe that air is a mixture is that freezing air slowly yields different liquids at different temperatures. Liquid nitrogen has a different boiling point than liquid oxygen. They also freeze at different temperatures.
An inguinal hernia is a protrusion of abdominal-cavity contents through the inguinal canal. Symptoms are present in about 66% of affected people. This may include pain or discomfort especially with coughing, exercise, or bowel movements. Often it gets worse throughout the day and improves when lying down.
Inguinal hernia is the most common surgical problem of childhood. It results from a small sac that comes through the inguinal ring that is normally open during fetal life and closes around the time of birth. For reasons we don't understand, it does not close in some infants.