Perpetual motion is motion of bodies that continues indefinitely. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics. These laws of thermodynamics apply even at very grand scales. For example, the motions and rotations of celestial bodies such as planets may appear perpetual, but are actually subject to many processes that slowly dissipate their kinetic energy, such as solar wind, interstellar medium resistance, gravitational radiation and thermal radiation, so they will not keep moving forever. Thus, machines that extract energy from finite sources will not operate indefinitely, because they are driven by the energy stored in the source, which will eventually be exhausted. A common example is devices powered by ocean currents, whose energy is ultimately derived from the Sun, which itself will eventually burn out. Machines powered by more obscure sources have been proposed, but are subject to the same inescapable laws, and will eventually wind down. The laws of thermodynamics apply to closed linear systems. In 2017 new states of matter, time crystals, were discovered which may allow for perpetual motion by bypassing the laws of thermodynamics, but since a time crystal is not a closed system, it does not constitute a perpetual motion machine in the traditional sense.

In Greek mythology, Persephone (; Greek: Περσεφόνη), also called Kore (; "the maiden"), is the daughter of Zeus and Demeter and is the queen of the underworld. Homer describes her as the formidable, venerable majestic princess of the underworld, who carries into effect the curses of men upon the souls of the dead. Persephone was married to Hades, the god of the underworld. The myth of her abduction represents her function as the personification of vegetation, which shoots forth in spring and withdraws into the earth after harvest; hence, she is also associated with spring as well as the fertility of vegetation. Similar myths appear in the Orient, in the cults of male gods like Attis, Adonis, and Osiris, and in Minoan Crete. Persephone as a vegetation goddess and her mother Demeter were the central figures of the Eleusinian mysteries, which promised the initiated a more enjoyable prospect after death. In some versions, Persephone is the mother of Zeus's sons Dionysus, Iacchus, or Zagreus. The origins of her cult are uncertain, but it was based on very old agrarian cults of agricultural communities. Persephone was commonly worshipped along with Demeter and with the same mysteries. To her alone were dedicated the mysteries celebrated at Athens in the month of Anthesterion. In Classical Greek art, Persephone is invariably portrayed robed, often carrying a sheaf of grain. She may appear as a mystical divinity with a sceptre and a little box, but she was mostly represented in the process of being carried off by Hades. In Roman mythology, she is called Proserpina, and her mother, Ceres.

Persian art or Iranian art has one of the richest art heritages in world history and has been strong in many media including architecture, painting, weaving, pottery, calligraphy, metalworking and sculpture. At different times, influences from the art of neighbouring civilizations have been very important, and latterly Persian art gave and received major influences as part of the wider styles of Islamic art. This article covers the art of Persia up to 1925, and the end of the Qajar dynasty; for later art see Iranian modern and contemporary art, and for traditional crafts see arts of Iran. Rock art in Iran is its most ancient surviving art. Iranian architecture is covered at that article. From the Achaemenid Empire of 550 BC–330 BC for most of the time a large Iranian-speaking state has ruled over areas similar to the modern boundaries of Iran, and often much wider areas, sometimes called Greater Iran, where a process of cultural Persianization left enduring results even when rulership separated. The courts of successive dynasties have generally led the style of Persian art, and court-sponsored art has left many of the most impressive survivals. In ancient times the surviving monuments of Persian art are notable for a tradition concentrating on the human figure (mostly male, and often royal) and animals. Persian art continued to place larger emphasis on figures than Islamic art from other areas, though for religious reasons now generally avoiding large examples, especially in sculpture. The general Islamic style of dense decoration, geometrically laid out, developed in Persia into a supremely elegant and harmonious style combining motifs derived from plants with Chinese motifs such as the cloud-band, and often animals that are represented at a much smaller scale than the plant elements surrounding them. Under the Safavid dynasty in the 16th century this style was used across a wide variety of media, and diffused from the court artists of the shah, most being mainly painters.

Iran (Persia) possesses an extraordinary treasure of royal jewelry, including a copious amount of mother-of-pearl from the Persian Gulf. The Iranian crown jewels are among the largest, most dazzling and valuable jewel collection in the world. The jewels are displayed in the vaults of the Central Bank of Iran in Tehran, and are one of the most appealing tourist attractions in Iran. Akik is also exported from Iran to various countries including the Indian subcontinent.

Persian weave is a method of weave used in jewelry and other art forms. When used for making chains, the Persian weave makes a dense chain with a ring pattern. Persian weaves works best with relatively large rings. The Persian family of weaves is a "rope" type weave based on a stacked ring orientation. In the construction of Persian weaves the rings become stacked and form pairs, resulting in a different grain and a thicker, tighter weave. Most Persian weaves have both left and right-handed versions.

A personal computer (PC) is a multi-purpose computer whose size, capabilities, and price make it feasible for individual use. PCs are intended to be operated directly by an end user, rather than by a computer expert or technician. Computer time-sharing models that were typically used with larger, more expensive minicomputer and mainframe systems, to enable them be used by many people at the same time, are not used with PCs. Early computer owners in the 1960s, invariably institutional or corporate, had to write their own programs to do any useful work with the machines. In the 2010s, personal computer users have access to a wide range of commercial software, free software ("freeware") and free and open-source software, which are provided in ready-to-run form. Software for personal computers is typically developed and distributed independently from the hardware or OS manufacturers. Many personal computer users no longer need to write their own programs to make any use of a personal computer, although end-user programming is still feasible. This contrasts with systems such as smartphones or tablet computers, where software is often only available through a manufacturer-supported channel, and end-user program development may be discouraged by lack of support by the manufacturer. Since the early 1990s, Microsoft operating systems and Intel hardware have dominated much of the personal computer market, first with MS-DOS and then with Windows. Alternatives to Microsoft's Windows operating systems occupy a minority share of the industry. These include Apple's macOS and free open-source Unix-like operating systems such as Linux. Advanced Micro Devices (AMD) provides the main alternative to Intel's processors.

A personal watercraft (PWC), also called water scooter, and comically a boatercycle, is a recreational watercraft that the rider sits or stands on, rather than inside of, as in a boat. PWCs have two style categories, first and most popular being a runabout or "sit down" where the rider uses the watercraft mainly sitting down, and the watercraft typically holds two or more people. The second style is a "stand-up", where the rider uses the watercraft standing up. The stand-up styles are built for one rider and are used more for doing tricks, racing, and use in competitions. Both styles have an inboard engine driving a pump-jet that has a screw-shaped impeller to create thrust for propulsion and steering. Most are designed for two or three people, though four-passenger models exist. Many of today's models are built for more extended use and have the fuel capacity to make long cruises, in some cases even beyond 100 miles.Personal watercraft are often referred by the trademarked brand names Jet Ski (referring to personal watercraft manufactured by Kawasaki), WaveRunner (referring to personal watercraft manufactured by Yamaha), or Sea-Doo (referring to personal watercraft manufactured by Bombardier). The United States Coast Guard defines a personal watercraft, amongst other criteria, as a jet-drive boat, less than 13 feet (4 m) in length, in order to exclude from that definition more conventional-sized jet-boats. There is a wide variety of "jetboats" many of which exceed 30–40 feet (9–12 m) in length.

A pet or companion animal is an animal kept primarily for a person's company, entertainment, or as an act of compassion such as taking in and protecting a hungry stray cat, rather than as a working animal, livestock, or laboratory animal. Popular pets are often noted for their attractive appearances, intelligence, and relatable personalities, or may just be accepted as they are because they need a home. Two of the most popular pets are dogs and cats. The technical term for a cat lover is an ailurophile, and for a dog lover, a cynophile. Other animals commonly kept include rabbits; ferrets; pigs; rodents, such as gerbils, hamsters, chinchillas, rats, and guinea pigs; avian pets, such as parrots, passerines, and fowl; reptile pets, such as turtles, alligators, crocodiles, lizards, and snakes; aquatic pets, such as fish, freshwater and saltwater snails, and frogs; and arthropod pets, such as tarantulas and hermit crabs. Small pets may be grouped together as pocket pets, while the equine and bovine group include the largest companion animals. Pets provide their owners (or "guardians") both physical and emotional benefits. Walking a dog can provide both the human and the dog with exercise, fresh air, and social interaction. Pets can give companionship to people who are living alone or elderly adults who do not have adequate social interaction with other people. There is a medically approved class of therapy animals, mostly dogs or cats, that are brought to visit confined humans, such as children in hospitals or elders in nursing homes. Pet therapy utilizes trained animals and handlers to achieve specific physical, social, cognitive or emotional goals with patients. Some scholars, ethicists and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and objectification of nonhuman animals.

Petra (Arabic: البتراء, Al-Batrāʾ; Ancient Greek: Πέτρα), originally known to the Nabataeans as Raqmu, is a historical and archaeological city in southern Jordan. The city is famous for its rock-cut architecture and water conduit system. Another name for Petra is the Rose City due to the color of the stone out of which it is carved. Petra is one of the New7Wonders of the World. Established possibly as early as 312 BC as the capital city of the Arab Nabataeans, it is a symbol of Jordan, as well as Jordan's most-visited tourist attraction. The Nabataeans were nomadic Arabs who took advantage of Petra's proximity to regional trade routes to establish it as a major trading hub. The Nabataeans are also known for their great ability in constructing efficient water-collecting methods in the barren deserts and their talent in carving structures into solid rocks. Petra lies on the slope of Jebel al-Madhbah (identified by some as the biblical Mount Hor) in a basin among the mountains which form the eastern flank of Arabah (Wadi Araba), the large valley running from the Dead Sea to the Gulf of Aqaba. It has been a UNESCO World Heritage Site since 1985. The site remained unknown to the Western world until 1812, when it was introduced by Swiss explorer Johann Ludwig Burckhardt. It was described as "a rose-red city half as old as time" in a Newdigate Prize-winning poem by John William Burgon. UNESCO has described it as "one of the most precious cultural properties of man's cultural heritage". Petra was named amongst the New7Wonders of the World in 2007 and was also chosen by the Smithsonian Magazine as one of the "28 Places to See Before You Die".

A petrol engine (known as a gasoline engine in American English) is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels. In most petrol engines, the fuel and air are usually mixed after compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.