The definition of Laughter is “…to express certain emotions, especially mirth or delight, by a series of spontaneous, usually unarticulated sounds often accompanied by corresponding facial and bodily movements.” The definition itself makes me want to laugh.
Scientists have a variety of theories about why people laugh. One theory is that laughter is related to surprise. A baby is startled and starts to cry, but instantly realizes there is no danger, so the cry turns to a laugh. Another similar theory is that we laugh when something is incongruous. In other words, we are confronted with two things that don’t fit together. That may be what caused Abraham to laugh in Genesis chapter 17.