Tin can telephone
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 were based on sound transmission through pipes or other physical media, and among the very earliest experiments were those conducted by the British physicist and polymath Robert Hooke from 1664 to 1685. From 1664 to 1665 Hooke experimented with sound transmission through a taut distended wire. An acoustic string phone is attributed to him as early as 1667.
The highly similar acoustic tin can telephone, or 'lover's phone', has also been known for centuries. It connects two diaphragms with a taut string or wire, which transmits sound by mechanical vibrations from one to the other along the wire (and not by a modulated electric current). The classic example is the children's toy made by connecting the bottoms of two paper cups, metal cans, or plastic bottles with tautly held string.
For a short period of time acoustic telephones were marketed commercially as a niche competitor to the electrical telephone, as they preceded the latter's invention and didn't fall within the scope of its patent protection. When Alexander Graham Bell's telephone patent expired and dozens of new phone companies flooded the marketplace, acoustic telephone manufacturers could not compete commercially and quickly went out of business. Their maximum range was very limited, but hundreds of technical innovations (resulting in about 300 patents) increased their range to approximately a half mile (800 m) or more under ideal conditions. An example of one such company was Lemuel Mellett’s 'Pulsion Telephone Supply Company' of Massachusetts, which designed its version in 1888 and deployed it on railroad right-of-ways, purportedly with a range of 3 miles (4.8 km).
In the centuries before tin cans and paper cups became commonplace, other cups were used and the device was sometimes called the "lovers' telephone". During the 20th century, it came into common use in preschools and elementary schools to teach children about sound vibration.
Sound waves are created as the air vibrates in response to a person's speech or other sounds. A second person's ear collects these sound waves and converts them into nerve impulses which their brain interprets as sound. In normal speech these waves travel through the air, but with a tin can telephone the waves are transmitted through an additional medium of cups and string.
When the string is pulled taut and someone speaks into one of the cans, its bottom acts as a diaphragm, converting the sound waves into longitudinal mechanical vibrations which vary the tension of the string. These variations in tension set up longitudinal waves in the string which travel to the second can, causing its bottom to vibrate in a similar manner as the first can, thus recreating the sound heard by the second person.
The signal can be directed around corners with the aid of a third can positioned on the apex of the corner. The string is threaded through the base of the third can so as to avoid coming into contact with the object around which the signal is to be directed. 
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- Preface to Micrographia (1665) «I have, by the help of a distended wire, propagated the sound to a very considerable distance in an instant». Micrographia - Extracts From The Preface
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- NPASS2 String Telephones, “Corner Busters” photos taken 7 October 2012.