A tin can telephone is a type of acoustic (non-electrical) speech-transmitting device made up of two tin cans, paper cups or similarly shaped items attached to either end of a taut string or wire. It is a form of mechanical telephony, where sound is converted into and then conveyed by vibrations along a liquid or solid medium, and then reconverted back to sound. == History == Before the invention of the electromagnetic telephone, there were mechanical acoustic devices for transmitting spoken words and music over a distance greater than that of normal speech. The very earliest mechanical telephones...

For the case of more than one variable, see Conic section or Quadratic form. In elementary algebra, a quadratic equation (from the Latin quadratus for "square") is any equation having the form where x represents an unknown, and a, b, and c represent known numbers such that a is not equal to 0. If a = 0, then the equation is linear, not quadratic. The numbers a, b, and c are the coefficients of the equation, and may be distinguished by calling them, respectively, the quadratic coefficient, the linear coefficient and the constant or free term. Because the quadratic equation involves only one unknown...