Dance is a type of art that generally refers to movement of the body, usually rhythmic and to music, performed in many different cultures and used as a form of expression, social interaction or presented in a spiritual or performance setting.

Dance may also be regarded as a form of nonverbal communication between humans, and is also performed by other animals (bee dance, patterns of behaviour such as a mating dance). Gymnastics, figure skating and synchronized swimming are sports that incorporate dance, while martial arts kata are often compared to dances. Motion in ordinarily inanimate objects may also be described as dances (the leaves danced in the wind).

Definitions of what constitutes dance are dependent on social, cultural, aesthetic, artistic and moral constraints and range from functional movement (such as folk dance) to virtuoso techniques such as ballet. Dance can be participatory, social or performed for an audience. It can also be ceremonial, competitive or erotic. Dance movements may be without significance in themselves, such as in ballet or European folk dance, or have a gestural vocabulary/symbolic system as in many Asian dances. Dance can embody or express ideas, emotions or tell a story.

Night or nighttime is the period of time between the sunset and the sunrise when the Sun is below the horizon. This occurs after dusk. The opposite of night is day (or "daytime" to distinguish it from "day" as used for a 24-hour period). The start and end points of time of a night vary based on factors such as season, latitude, longitude and timezone.

At any given time, one side of the planet Earth is bathed in light from the Sun (the daytime) and the other side of the Earth is in the shadow caused by the Earth blocking the light of the sun. This shadow is what we call the darkness of night. Natural illumination is still provided by a combination of moonlight, planetary light, starlight, diffuse zodiacal light, gegenschein, and airglow. In some circumstances, bioluminescence, aurorae and lightning can provide some illumination. The glow provided by artificial illumination is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems.

In physics, energy (Greek: ἐνέργεια energeia "activity, operation") is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems. Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

The total energy contained in an object is identified with its mass, and energy (like mass), cannot be created or destroyed. When matter (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the mass of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into work, on other systems. Energy, like mass, is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in many fields other units, such as kilowatt-hours and kilocalories, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through.