"Lover's Telephone"

History:
Invented in 1665 by Robert Hooke, an English philosopher also known for first applying the word 'cell' to the basic unit of life. Considered one of the most important scientists of the 17th century, Hooke discovered a way to "to hear one speak through a wall a yard thick...I have, by the help of a distended wire, propagated the sound to a very considerable distance in an instant."
How it works:
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 waves in the string which travel to the other can, causing its bottom to vibrate in a similar manner as the first can, thus recreating the sound.

For more on the history of telecommunications, click here