Just as antibody production is connected to both exercise and happiness, so is endorphin production. Endorphins are chemicals—produced in various parts of our body—that are able to cross through the gaps between neurons in order to pass along a message from one to the next. There are many different kinds of endorphins, and much remains to be learned about their different purposes and functions. One thing that is known for certain about endorphins is their ability to make us feel oh-so-good, or the so-called "endorphin rush." We can see this in the formation of the word "endorphin." It consists of two parts: endo- and -orphin; these are short forms of the words endogenous and morphine, intended to mean "a morphine-like substance originating from within the body." When our body is subjected to certain stimuli like sex, exercise, food, or pain, our brain calls for endorphins, and the cells throughout our body that contain them answer the call. When endorphins lock into special receptor cells, they block the transmission of pain signals and also produce a euphoric feeling—exactly like what morphine can do to us. Endorphins act as both a painkiller and as a pleasure provider for our body's reward system. When we hurt ourselves, we may get a big dose of endorphins to ease the pain. We may also get an endorphin rush from talking to a stranger, eating a satisfying meal, or being exposed to ultraviolet light. Everyone has different amounts of endorphins, and what may trigger an endorphin rush for one person could very well fail to do so for someone else. The satisfaction level of the endorphin rush with which our body is tuned in lets us know we have had enough—and convinces us to do it again sometime soon. Exercise stimulates endorphin production as well, but for a different reason. Some people are probably more familiar with the term "runner's high," which refers to the feelings of excitement and satisfaction one sometimes gets when exercising. Researchers have found that light-to-moderate weight training or cardiovascular exercise doesn't produce endorphins, only heavy weights or training that involves jumping or other anaerobic body moves. When our body crosses over from an aerobic state to an anaerobic state, it's suddenly operating without enough oxygen to satisfy the muscles and cells screaming out for it. This is when the runner's high, or endorphin rush, occurs.