Laughter is an auible expression or appearance of merriment or amusement or an inward feeling of joy and pleasure (laughing on the inside). It may ensue (as a physiological reaction) from jokes, tickling and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter. Strong laughter can sometimes bring an onset of tears or even moderate muscular pain. Laughter is a part of human behaviour regulated by the brain. It helps humans clarify their intentions in social interaction and provides an emotional context to conversations. Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions with others. Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback. This may account in part for the popularity of laugh tracks in situation comedy television shows.