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The Magdeburg hemispheres are a pair of large copper hemispheres, with mating rims. They were used to demonstrate the power of atmospheric pressure. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart by teams of horses. The Magdeburg hemispheres were invented by German scientist and mayor of Magdeburg, Otto von Guericke, to demonstrate the air pump that he had invented, and the concept of atmospheric pressure. The first artificial vacuum had been produced a few years earlier by Evangelista Torricelli, and had inspired Guericke to design the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves. The hemispheres became popular in physics lectures as an illustration of the strength of air pressure, and are still used in education. A pair of the original hemispheres are preserved in the Deutsches Museum in Munich.

Space Infrastructure Servicing (SIS) is a spacecraft being developed by Canadian aerospace firm MacDonald, Dettwiler and Associates to operate as a small-scale in-space refueling depot for communication satellites in geosynchronous orbit.

In June 2017, SSL announced its first commercial customer, Luxembourg-based satellite owner/operator SES S.A.

The Banach–Tarski paradox is a theorem in set-theoretic geometry, which states the following: Given a solid ball in 3‑dimensional space, there exists a decomposition of the ball into a finite number of disjoint subsets, which can then be put back together in a different way to yield two identical copies of the original ball. Indeed, the reassembly process involves only moving the pieces around and rotating them without changing their shape. However, the pieces themselves are not "solids" in the usual sense, but infinite scatterings of points. The reconstruction can work with as few as five pieces.

A stronger form of the theorem implies that given any two "reasonable" solid objects (such as a small ball and a huge ball), the cut pieces of either one can be reassembled into the other. This is often stated informally as "a pea can be chopped up and reassembled into the Sun" and called the "pea and the Sun paradox".

The reason the Banach–Tarski theorem is called a paradox is that it contradicts basic geometric intuition. "Doubling the ball" by dividing it into parts and moving them around by rotations and translations, without any stretching, bending, or adding new points, seems to be impossible, since all these operations ought, intuitively speaking, to preserve the volume. The intuition that such operations preserve volumes is not mathematically absurd and it is even included in the formal definition of volumes. However, this is not applicable here because in this case it is impossible to define the volumes of the considered subsets. Reassembling them reproduces a volume, which happens to be different from the volume at the start.

Unlike most theorems in geometry, the proof of this result depends in a critical way on the choice of axioms for set theory. It can be proven using the axiom of choice, which allows for the construction of non-measurable sets, i.e., collections of points that do not have a volume in the ordinary sense, and whose construction requires an uncountable number of choices.

It was shown in 2005 that the pieces in the decomposition can be chosen in such a way that they can be moved continuously into place without running into one another.

Kugelmugel, officially the Republic of Kugelmugel (German: Republik Kugelmugel), is a spherical art object located in Vienna, Austria.

It came about as the result of the artist Edwin Lipburger constructing the 8 meter diameter spherical object without permissions from the authorities in Austria. After the dispute between the artist and authorities, the artist declared it a micronation, and it was eventually granted asylum by the then-mayor Helmut Zilk in Vienna where it is housed in the Prater park.

The 'Republic' is currently administered by Linda Treiber as president.

The secretary problem is a problem that demonstrates a scenario involving optimal stopping theory. The problem has been studied extensively in the fields of applied probability, statistics, and decision theory. It is also known as the marriage problem, the sultan's dowry problem, the fussy suitor problem, the googol game, and the best choice problem.

The basic form of the problem is the following: imagine an administrator who wants to hire the best secretary out of ${\displaystyle n}$ rankable applicants for a position. The applicants are interviewed one by one in random order. A decision about each particular applicant is to be made immediately after the interview. Once rejected, an applicant cannot be recalled. During the interview, the administrator gains information sufficient to rank the applicant among all applicants interviewed so far, but is unaware of the quality of yet unseen applicants. The question is about the optimal strategy (stopping rule) to maximize the probability of selecting the best applicant. If the decision can be deferred to the end, this can be solved by the simple maximum selection algorithm of tracking the running maximum (and who achieved it), and selecting the overall maximum at the end. The difficulty is that the decision must be made immediately.

The shortest rigorous proof known so far is provided by the odds algorithm (Bruss 2000). It implies that the optimal win probability is always at least ${\displaystyle 1/e}$ (where e is the base of the natural logarithm), and that the latter holds even in a much greater generality (2003). The optimal stopping rule prescribes always rejecting the first ${\displaystyle \sim n/e}$ applicants that are interviewed and then stopping at the first applicant who is better than every applicant interviewed so far (or continuing to the last applicant if this never occurs). Sometimes this strategy is called the ${\displaystyle 1/e}$ stopping rule, because the probability of stopping at the best applicant with this strategy is about ${\displaystyle 1/e}$ already for moderate values of ${\displaystyle n}$. One reason why the secretary problem has received so much attention is that the optimal policy for the problem (the stopping rule) is simple and selects the single best candidate about 37% of the time, irrespective of whether there are 100 or 100 million applicants.

The Hurrian songs are a collection of music inscribed in cuneiform on clay tablets excavated from the ancient Amorite-Canaanite city of Ugarit, a headland in northern Syria, which date to approximately 1400 BCE. One of these tablets, which is nearly complete, contains the Hurrian hymn to Nikkal (also known as the Hurrian cult hymn or A Zaluzi to the Gods, or simply h.6), making it the oldest surviving substantially complete work of notated music in the world. While the composers' names of some of the fragmentary pieces are known, h.6 is an anonymous work.

Metrecal was a brand of diet foods introduced in the early 1960s. Though its products were criticized for their taste, which newer varieties of flavor tried to improve upon later, it attained a niche in the popular culture of the time. Created and marketed initially by C. Joseph Genster of Mead Johnson & Company, it was eventually replaced in the market by competitors such as SlimFast and lost popularity because it was linked to deaths.

Air raid shelters, also known as bomb shelters, are structures for the protection of non-combatants as well as combatants against enemy attacks from the air. They are similar to bunkers in many regards, although they are not designed to defend against ground attack (but many have been used as defensive structures in such situations).

Prior to World War II, in May 1924, an Air Raid Precautions Committee was set up in the United Kingdom. For years, little progress was made with shelters because of the apparently irreconcilable conflict between the need to send the public underground for shelter and the need to keep them above ground for protection against gas attacks. In February 1936 the Home Secretary appointed a technical Committee on Structural Precautions against Air Attack.

By November 1937, there had only been slow progress, because of a serious lack of data on which to base any design recommendations and the Committee proposed that the Home Office should have its own department for research into structural precautions, rather than relying on research work done by the Bombing Test Committee to support the development of bomb design and strategy. This proposal was eventually implemented in January 1939.

During the Munich crisis, local authorities dug trenches to provide shelter. After the crisis, the British Government decided to make these a permanent feature, with a standard design of precast concrete trench lining. Unfortunately these turned out to perform very poorly. They also decided to issue free to poorer households the Anderson shelter, and to provide steel props to create shelters in suitable basements.

The bus factor is a measurement of the risk resulting from information and capabilities not being shared among team members, derived from the phrase "in case they get hit by a bus." It is also known as the bread truck scenario, lottery factor, truck factor, bus/truck number, or lorry factor.

The concept is similar to the much older idea of key person risk, but considers the consequences of losing key technical experts, versus financial or managerial executives (who are theoretically replaceable at an insurable cost). Personnel must be both key and irreplaceable to contribute to the bus factor; losing a replaceable or non-key person would not result in a bus-factor effect.

The term was first applied to software development, where a team member might create critical components by crafting code that performs well, but which also is unavailable to other team members, such as work that was undocumented, never shared, encrypted, obfuscated, unpublished, or otherwise incomprehensible to others. Thus a key component would be effectively lost as a direct consequence of the absence of that team member, making the member key. If this component was key to the project's advancement, the project would stall.

Claude Émile Jean-Baptiste Litre is a fictional character created in 1978 by Kenneth Woolner of the University of Waterloo to justify the use of a capital L to denote litres.

The International System of Units usually only permits the use of a capital letter when a unit is named after a person. The lower-case character l might be difficult to distinguish from the upper-case character I or the digit 1 in certain fonts and styles, and therefore both the lower-case (l) and the upper-case (L) are allowed as the symbol for litre. The United States National Institute of Standards and Technology now recommends the use of the uppercase letter L, a practice that is also widely followed in Canada and Australia.

Woolner perpetrated the April Fools' Day hoax in the April 1978 issue of "CHEM 13 News", a newsletter concerned with chemistry for school teachers. According to the hoax, Claude Litre was born on 12 February 1716, the son of a manufacturer of wine bottles. During Litre's extremely distinguished fictional scientific career, he purportedly proposed a unit of volume measurement that was incorporated into the International System of Units after his death in 1778.

The hoax was mistakenly printed as fact in the IUPAC journal Chemistry International and subsequently retracted.