The Numbers API is a site to explore facts about numbers. Enter a number and receive a random fact, such as "15 is the number of days in each of the 24 cycles of the Chinese calendar." In addition to viewing them on the site, the Numbers API is expected to be used programmatically. Send a request to the API and it returns a plain text string. Add a callback and that string is wrapped in a JSONP callback function.
In addition to simple number facts, the site also allows developers to specify math facts about numbers, as well as "on this date" type of functionality.