A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile. Among humans, a smile expresses delight, sociability, happiness, joy, or amusement. It is distinct from a similar but usually involuntary expression of anxiety known as a grimace. Although cross-cultural studies have shown that smiling is a means of communication throughout the world, there are large differences among different cultures, religions, and societies, with some using smiles to convey confusion, embarrassment or awkwardness.

Happiness is a positive and pleasant emotion, ranging from contentment to intense joy. Moments of happiness may be triggered by positive life experiences or thoughts, but sometimes it may arise from no obvious cause. The level of happiness for longer periods of time is more strongly correlated with levels of life satisfaction, subjective well-being, flourishing and eudaimonia. In common usage, the word happy can be an appraisal of those measures themselves or as a shorthand for a "source" of happiness. As with any emotion, the precise definition of happiness has been a perennial debate in philosophy.