Addiction: Central nervous system (CNS) stimulants are addictive, even if they are not abused. Many factors, including the patient's personal history, the type of stimulant that is being taken, the dose of the drug, and duration of use, influence how likely an individual is to become addicted.
The source of all waves - mechanical or electromagnetic - is something that is vibrating. Matthew Donovan.
Huygen's principle of elementary wave centres. -every point within a wave front itself is a source of a wave propagating in all directions. - apparent bending of waves around small obstacles and the spreading out of waves past small openings. Example.