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We laugh in joy and cry over pain, right? Well, it turns out that it's not so black-and-white. Although not fully understood, we can laugh until we cry.

One explanation for this is that both laughing and crying happen during times of high emotional arousal, University of Maryland at Baltimore psychologist Robert Provine told Prevention.com.

There's also evidence that the same part of the brain is responsible for both crying and laughing. In fact, lesions on certain regions of the brain have been linked to a syndrome called pathological laughter and crying (PLC), which is characterized by uncontrollable outbursts of these two expressions. The fact that the same lesion causes both tears and laughter suggests they are linked somehow, according to a PBS special, "The Adult Brain."

Another explanation points to pressure being put on the tear ducts as a result of a body-shaking, vigorous laugh. These tears are considered reflex tears, because they result from external factors, such as an airborne irritant or the wind, and not due to emotion.

Emotional tears have a different chemical makeup than the reflex teardrops, according to research by William Frey, professor at the University of Minnesota. The emotion-driven water drops contain more of certain hormones, including leucine enkephalin, a natural painkiller.

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