Which HTTP methods are idempotent?

Given a REST API that is capable of responding only with JSON, what is the appropriate way to respond to a request with header `Accept: application/xml; charset=utf-8`?

The REpresentational State Transfer (REST) architectural style is based on the HTTP protocol.

In the context of a web service, a RESTful API should...

A RESTful Web Service URI should map to __________________?

When you wish to read a collection of “Foo” via a RESTful API, which is most appropriate?