1. Skin
   Bone
2. Muscle
   Veins

The human body has three different kinds of muscles. Cardiac muscle is what makes up the heart, smooth muscle is found on the outside of internal organs, and skeletal muscle is linked to the skeleton.

1. Floatness
   Buoyancy
2. Aquatic rebound
   Density opposition

Buoyancy is created when a fluid exerts a certain amount of force on an object that has been immersed in it. The idea is that because pressure increases with depth, there will always be an upward thrust on objects, since there is greater force at the bottom.

1. Troposphere
   Mesosphere
2. Ozone layer
   Thermosphere

Without the presence of the ozone layer, more of the sun's harmful UV-B rays would reach the Earth's surface. This would result in higher rates of cancer and an increased global temperature.

1. Mantle energy
   Geothermal energy
2. Magmonic energy
   Geoheat energy

Geothermal energy is an excellent alternative for those looking for a sustainable and clean method to provide power. Heat from below the Earth's surface is used to create steam, which rotates a turbine, creating energy.

1. Anemia
   Pellagra
2. Rickets
   Scurvy

Due to the limited food supply available on ships, pirates and sailors dealt with scurvy. The disease caused their teeth to fall out, their gums to bleed and their hair to thin. Without treatment, people usually perished.

1. Infinite zero
   Absolute zero
2. Final zero
   Theoretical zero

Absolute zero is the temperature in the universe at which atoms completely stop moving. Absolute zero is defined by the Kelvin scale, but translated, it is approximately -460 degrees Fahrenheit or -273 degrees Celsius.

1. Yield strength
   Tensile strength
2. Mechanics of materials
   Fatigue strength

Tensile strength is the measure of the force that is needed to break an object. For example, if a rope is being pulled on, the moment before it breaks would reveal its tensile strength.

1. Potential energy
   Kinetic energy
2. Thermal energy
   Chemical energy

Kinetic energy is defined as the energy of motion. In order for energy to be converted from potential to kinetic, a force must act upon an object so as to put the object in motion.

1. Period
   Magnitude
2. Wavelength
   Peak distance

Wavelength can also be found by comparing the distance between successive waves' troughs and zero crossings. Wavelength is influenced by the medium it is traveling through, whether it's water, air or something else entirely.

1. DNA
   Thymine
2. Nucleic acids
   Amino acids

Our bodies receive essential, conditionally essential and nonessential amino acids from the protein that we eat. These organic compounds repair muscle tissue, promote growth, help with digestion and assist in many other bodily functions.

1. Tears
   Tectonic shifts
2. Geological rifting
   Faults

Faults can be found across the globe — they are the most common cause of earthquakes. One of the most active fault areas is the Ring of Fire, which is found along the basin of the Pacific Ocean.

1. Electron
   Proton
2. Nucleus
   Quark

Electrons are necessary for producing a great number of phenomena in the physical world. Without electrons, we would not have thermal conductivity, magnetism, electricity or even chemistry!

1. Planck's Constant
   Parsec
2. Mole
   Bohr radius

Prior to its official naming, three options had been proposed for the vast distance being discussed. Frank Watson Dyson had put forth the name "astron," while other scientists had come up with "siriometer" and "parsec." Parsec was ultimately chosen.

1. Newton
   Mole
2. Ampere
   Joule

A joule is a unit of measurement for energy, and it is equal to the work necessary to produce one watt of power over the span of one second. Based on this, a joule can also be measured as a watt-second.

1. Carbon
   Helium
2. Iron
   Potassium

Carbon is a very adaptable element and can be found in a number of compounds. In fact, it is part of more compounds than all the rest of the elements put together! Some of its more common forms are charcoal, graphite and diamond.

1. Yeast
   Prokaryotes
2. Entamoeba coli
   Archae

Yeast is very easy to find in the world. It naturally occurs on the exterior of some fruits, like peaches and grapes. Yeast can also be found on the human body, living between a person's toes and other places.

1. Neuronian extensions
   Dendrites
2. Axon
   Soma

A neuron, or nerve cell, is made up of the soma, axons, axon terminals and dendrites. A neuron's dendrites communicate with other neurons, bringing in information through electrochemical stimulation.

1. Noble gas
   Stable isotope
2. Radioactive isotope
   Unstable isotope

Cobalt-60 is a radioactive isotope that plays a large role in cancer treatment. Medical professionals can use the isotope to kill harmful cells or even control their growth, sometimes eliminating the need for surgery.

1. Penumbra
   Antumbra
2. Umbra
   The opaque region

Shadows are made up of three different parts, called the antumbra, the penumbra and the umbra, which is the darkest part of a shadow. The umbra is where any light is completely blocked out by another object.

1. Supernova
   Galaxy
2. Dark matter
   Nebula

Nebulae can form when a star dies and explodes or in an area where new stars are trying to form. One well-known nebula is the Crab Nebula, which was formed when the Taurus constellation suffered a supernova.

1. Granite
   Molten mantle
2. Magma
   Brucite

Magma is molten rock, found beneath the Earth's surface. When magma cools below ground, it forms plutonic rocks, such as gabbro, which is quite common in the Earth's deep oceanic crust.

1. Quark
   Lepton
2. Fermion
   Higgs boson

Up until 2013, the Higgs boson was only a theoretical particle in the Standard Model of particle physics. Its existence was confirmed with the Large Hadron Collider located at CERN, near Geneva, Switzerland.

1. Quarks
   Leptons
2. Gluons
   Gauge bosons

Despite their perceived existence, quarks cannot be observed directly or as a single quark. They can only be detected when they have joined together to form hadrons. A quark carries a charge, known as its "color."

1. Astronomical horizon
   Zenith
2. True horizon
   Nadir

The term "zenith" is used to describe the point directly overhead. The sun is at its zenith above you if a pencil poked straight into the ground casts absolutely no shadow. The sun's zenith passage relates to the Mayan calendar.

1. Speed
   Force
2. Momentum
   Time

The initial definition of momentum began with work done by the French scientist Rene Descartes. From there, the English scientist Sir Isaac Newton took things further and developed his laws of motion, which consider momentum.

1. Cyanobacteria
   Coccolithophores
2. Brown algae
   Diatoms

Diatoms are found in various water sources and soils around the world. They are a great fertilizer once they die, as their shells erode to dust which is scattered across nearby land by the wind.

1. Joule
   Heat unit
2. Latent heat exchange
   Calorie

The word "calorie" is not seen as often as "joule" or "kilojoule" in scientific texts these days, given the modern way of measuring temperature increase through joules. One calorie has the same amount of energy as approximately 4.18 joules.

1. Valency
   Oxidation
2. Molecular pull
   Atomic force

Valency refers to the potential activity of valence electrons. These are the electrons found in an element's outermost shell that can be used to create bonds with another element's particles.

1. Igneous rock
   Sedimentary rock
2. Metamorphic rock
   Cosmogenous rock

Igneous rocks are formed from lava or magma that cools and solidifies. If they form below the Earth's surface, they are considered intrusive igneous rocks; if they form above the surface, they're extrusive.

1. Phloem
   Flower
2. Stamen
   Xylem

The xylem is responsible for getting water from the roots to other parts of the plant, in addition to distributing nutrients. When a plant loses moisture through functions such as photosynthesis and transpiration, the xylem works to replace it.

1. Latent heat
   Sensible heat
2. Radiation
   Specific heat

Latent heat can cause phase changes within a substance. This can mean turning something from a solid to a liquid to a gas and vice versa. This heat does not affect the overall temperature of the object.

1. Watt
   Ohm
2. Alternating current
   Apparent power

"Ohm" is the term used to describe the electrical resistance between two points. The unit of measurement was named after Georg Simon Ohm, who put together many theories of electricity.

1. Spring tide
   Neap tide
2. Diurnal tide
   Semi-diurnal tide

Spring tides happen when the gravitational pull of the moon and sun are closely aligned. Because of this extra gravitational pull on the Earth's tides, spring tides include high tides that are higher and low tides that go below their average.

1. Phytoplankton
   Zooplankton
2. Diatoms
   Dinoflagellates

Many zooplankton can only be seen with the help of a microscope, but jellyfish can be observed with the naked eye. There have even been jellyfish specimens collected that measure more than 120 feet long!

1. Particles
   Saturn's moons
2. Lava
   Energy field types

Lava is able to travel long distances, despite being thicker than water. Lava that is more viscous tends to come from explosive eruptions, while lava with low viscosity tends to flow from a volcano like a river.