Long thought to cause feelings of fear or awe, infrasound is sound that is able to cover long distances with little dissipation at a frequency too low to be detected by the human ear. It is created by natural phenomena such as ocean waves and earthquakes and can be artificially produced by explosions or machines. Infrasound is used by some animals to communicate over long distances and by predators to “freeze” their prey. Why do some scientists believe infrasound is related to ghost sightings? Discuss
Source: The Free Dictionary
Used in machine guns since World War I, tracer ammunition contains modified bullets packed with a pyrotechnic charge that ignites upon firing, illuminating the bullets. This technology enables a shooter to check a bullet’s trajectory and adjust his or her aim accordingly. Since the tracer path can also betray a shooter’s position, many modern tracers are designed to become visible only once they have traveled a set distance. Why were tracers especially useful against Zeppelins in World War I? 
A molecular machine is a minute mechanism consisting of molecular components that perform mechanical-like movements in response to specific stimuli. Chemists have synthesized a number of simple molecular machines, including molecular propellers and molecular motors, the latter of which are powered by light or reactions with other molecules and are capable of unidirectional rotation. Far more complex biological versions of these artificial nanomachines are found in living cells. What do they do? 
On November 20, 1979, approximately 200 armed Islamic fundamentalist dissidents opposed to the Saudi ruling family took over Islam’s holiest place, the Al-Masjid al-Haram in Mecca, Saudi Arabia. After 127 Saudi Arabian National Guard members were killed in an unsuccessful attempt to regain control, the nervous Saudi government called in French and Pakistani forces, which retook the shrine in a battle that left approximately 250 dead and 600 wounded. What happened to the surviving militants? 
Travelers who fail to familiarize themselves with a region’s customs before visiting that area risk inadvertently offending or insulting their hosts. For example, in Iraq and Iran, the “thumbs up” gesture—a Western expression of approval—is considered an offensive insult. In the Middle East, where Islam is the predominant religion, modesty and hospitality are strongly valued traits, but their practice varies across ethnic groups. How do Middle Easterners show their respect for the elderly? 
Intellectual property is an umbrella term for the various legal entitlements associated with certain names, literary works, recorded media, and inventions. The holders of such entitlements may exercise various exclusive rights in relation to the use of their intellectual property. The concept is controversial because it implies that intellectual works—so-called products of the mind—are analogous to physical property. What is the difference between a copyright, a trademark, and a patent? 
In mathematics, a perfect number is an integer that is the sum of all of its proper, positive divisors not including the number itself. The first perfect number is 6, because 1, 2, and 3 are its proper, positive divisors and 1+2+3=6. Euclid, a Hellenistic mathematician living in the third century BCE, discovered a formula that identifies the first four perfect numbers. What is the second number in this set? 
In one of the first trials followed by the public through daily newspaper reports, Frenchwoman Marie-Fortunée Lafarge, who stood accused of poisoning her husband with arsenic, was found guilty of murder. Her conviction in 1840 was the first to be based heavily on direct forensic toxicological evidence. During the trial, local experts presented contradictory testimony regarding the presence of arsenic in the victim’s body, and it was ultimately exhumed and tested using what new detection method? 
In physics, space-time is a four-dimensional continuum consisting of three dimensions in space and one dimension in time. This mathematical model, which is an integral part of Einstein’s relativity theory, combines space and time into a single continuum that allows physicists to more uniformly describe the workings of the universe. The merger of space and time is often represented as a measure of distance expressed in units of time. What novelists have referenced the topic of space-time? 
Both DNA and plastic, two seemingly unrelated materials, are classified as polymers, compounds of usually high molecular weight consisting of a number of structural units linked together by covalent bonds. A diverse assortment of natural and synthetic materials comprises the class of polymers, including hair, polyester, and shellac. Polymers are even used in the production of microprocessor parts. What is the process that produces polymers called?