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Sound is an interruption of mechanical energy that propagates through matter as a wave. Sound is characterized by the properties of sound waves which are frequency, wavelength, period, amplitude and velocity or speed. Noise and sound often mean the same thing; when they differ, a noise is a useless sound. In science and engineering, noise is an undesirable component that obscures a signal.

Humans perceive sound by the sense of hearing. By sound, we usually mean the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider description of sound that includes low and high frequency vibrations in air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases, liquids and solids. The matter that supports the sound is called the medium. Sound propagates as waves of irregular pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate. The scientific study of sound is called acoustics.

Mean sea level pressure is the pressure at sea level or the station pressure condensed to sea level assuming an isothermal layer at the station temperature. This is the pressure usually given in weather reports on radio, television, and newspapers. When barometers in the home are set to match the local weather reports, they measure pressure condensed to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the usual range of fluctuations in pressure is the same for everyone. The pressures which are measured high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tool. The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.

QNH barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield.

QFE barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield. In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.

Multicellular organisms are those organisms consisting of more than one cell, and having differentiated cells that achieve specialized functions. Most life that can be seen with the naked eye is multicellular, as are all animals and plants. A group of similarly differentiated cells performing a function in a multicellular organism is known as a tissue. Although some single-celled organisms, such as mycobacteria, have differentiated cells, the differentiation is less dramatic than that usually found in multicellular organisms.

A multicellular organism can work on a number of levels. For instance, a sponge works on a cellular level as the different cells are almost like many separate organisms co-operating jointly for a common goal. This is illustrated by the fact you can put a sponge in a blender and the cells of the sponge will aggregate again to form another sponge. They have no tissues, organs or organ systems which require a higher level of organization. More advanced organisms such as jellyfish, coral and sea anemones work on a tissue level of organization, as their cells can aggregate to form different tissues. For instance, a jellyfish has an outer epidermis and an inner gastrodermis, an example of different tissues to serve a different purpose.

Stone Age rock shelters with paintings at Bhimbetka in the state of Madhya Pradesh are the earliest known traces of human life in India. The first known permanent settlements appeared over 9,000 years ago and gradually developed into the Indus Valley Civilization, dating back to 3300 BCE in western India. It was followed by the Vedic Civilization which laid the foundations of Hinduism and other cultural aspects of early Indian society. From around 550 BCE, many independent kingdoms and republics known as the Mahajanapadas were established across the country laying the foundations of ancient India.

The empire built by the Maurya dynasty under Emperor Ashoka the Great united most of modern Southern Asia except the Dravidian kingdoms in the south. From 180 BCE, a series of invasions from Central Asia followed including the Indo-Greeks, Indo-Scythians, Indo-Parthians and Kushans in the north-western Indian Subcontinent. From the third century CE, the Gupta dynasty oversaw the period referred to as ancient India's "Golden Age." While the north had larger, fewer kingdoms, in the south there were several dynasties such as the Chalukyas, Rashtrakutas, Hoysalas, Cheras, Cholas, Pallavas and Pandyas in different times and regions.

While some sports, such as fishing, have been viewed as primarily recreational, most sports are considered competitive. The greater part involve competition between two or more persons. For example, in a game of basketball, two teams compete against one another to determine who can score the most points. While there is no set reward for the winning team, many players gain an inner sense of pride. In addition, extrinsic rewards may also be given. Athletes, besides competing against other humans, also compete against nature in sports such as whitewater kayaking or mountain hiking, where the goal is to reach a destination, with only natural barriers impeding the process.

While professional sports have been usually viewed as intense and extremely competitive, recreational sports, which are often less intense, are considered a healthy option for the competitive urges in humans. Sport provides a relatively safe venue for converting unbridled competition into safe competition, because sports competition is restrained. Competitive sports are governed by codified rules agreed upon by the participants. Violating these rules is considered to be unfair competition.