The reasons we laugh, including "contagious" laughter, may be products of evolution. 

    Natural laughter is a two-part, spontaneous, response to humor, that has physiological, psychological, and physical benefits. 

    Most agree that we laugh when we find something to be humorous, yet different reasons exist for what we find to be humorous. Additionally, different things are humorous to us at different stages of life. 

    Laughter, a physiological response to humor, can be broken down into two parts. 

    The first is a set of gestures, and the second is the production of sound. The brain forces to conduct both responses simultaneously. From a physiological standpoint, a "sensor" (探测器)in the brain responds to laughter by triggering(刺激) other neural circuits in the brain, which, in turn, generate more laughter. 

    Oddly enough, laughter is an orderly response, and almost occurs "spontaneously" during pauses at the end of phrases, earning it the name the punctuation effect. Human beings are the only species capable of laughter, and the average adult does so approximately 17 times per day. 

    Good health is one of the many benefits of laughter. Laughter reduces our stress levels by reducing the level of stress hormones, and also helps us cope with serious illnesses. 

    Physiologically, laughter promotes healing, by lowering the blood pressure, and by increasing the vascular blood flow and the oxygenation of the blood. 

    Physical fitness stemming from laughter is a benefit known to few. Scientists estimate that laughing 100 times is equivalent to a 10-minute workout on a rowing machine, or to 15 minutes on a stationary exercise bike. The mere act of laughing exercises the diaphragm(胸隔膜), as well as the abdominal(腹部的), respiratory(呼吸的), facial, leg, and back muscles. 

    Another benefit of laughter is that it improves our over-all mental health. Pent up negative emotions, such as anger, fear, and sadness, can cause biochemical changes in our bodies that can produce a harmful effect. 

    Laughter provides a harmless outlet(发泄) for these negative emotions, and provides a coping mechanism for dealing with difficult or stressful situations.