Is it possible to be invisible?

amphipod: An order of water-dwelling crustaceans related to lobsters, shrimp, and krill. Amphipods vaguely resemble shrimp. Most types are less than one centimeter (0.4 inch) long. 

cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell. 

component: Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe). 

compound: (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H₂O. 

contract: To activate muscle by allowing filaments in the muscle cells to connect. The muscle becomes more rigid as a result.  

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size, mental maturity or sometimes even shape.  

ecologist: A scientist who works in a branch of biology that deals with the relations of organisms to one another and to their physical surroundings. 

fiber: Something whose shape resembles a thread or filament. 

hemoglobin: A molecule that binds to oxygen in the blood, carrying it around to tissues. 

liver: An organ of the body of animals with backbones that performs a number of important functions. It can store fat and sugar as energy, break down harmful substances for excretion by the body, and secrete bile, a greenish fluid released into the gut, where it helps digest fats and neutralize acids. 

melanin: A family of pigments found in all types of animals. They are responsible for the dark coloring in such things as feathers, hair, fur, skin and scales. 

molecule: A group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in air is made of two bound oxygen atoms (O₂). Water is made of two hydrogen atoms and one oxygen atom (H₂O). 

muscle: A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue. 

oxygen: A gas that makes up about 21 percent of Earth’s atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism). 

physicist: A scientist who studies the nature and properties of matter and energy. 

predator: (adjective: predatory) A creature that preys on other animals for most or all of its food. 

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Antibodies, hemoglobin and enzymes are all examples of proteins. Medicines frequently work by latching onto proteins. 

reflective: Adjective that refers to the ability of something to reflect light strongly. Such objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake. 

refraction: The change in direction of light (or any other wave) as it passes through some material. For example, the path of light leaving water and entering air will bend, making partially submerged objects to appear to bend at the water’s surface. 

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty. 

seawater: The salty water found in oceans. 

species: A group of similar organisms capable of producing offspring that can survive and reproduce. 

sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star. 

tissue: Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. 

transition: The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes or converts into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another.  

transparent: Allowing light to pass through so that objects behind can be distinctly seen. Or information that lets others understand the otherwise hidden people, processes and impacts behind some product, action or proposal. 

vein: (in zoology) Part of a body’s circulation system. These tubes usually carry blood toward the heart. 

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.