Besieged by Prussians in 1870, Paris found a clever way to get mail to the outside world. For 20 centimes you could write a letter on a thin piece of green paper; these were collected and sent hopefully upward on unguided mail balloons. Each 4-gram postcard carried an address; the Parisians hoped that the balloons would drift to earth somewhere and that whoever found the messages would forward them.

It worked. During the four-month siege they sent up 65 balloons, and only two went missing.