If you're running an earlier version of Ruby than Ruby 1.9.2, first install the gem with the following command:

gem install json

Note that Ruby's implementation is in C, so installing the gem may require a C compiler. If you don't have one installed on your system, you can install the pure Ruby implementation of the gem using the following command:

gem install json_pure

Regardless of whether you need to install the gem or not, you'll need to include it in your code. To do this, include both rubygems and json or json/pure, depending on which gem you installed; do this using require, like this:

require 'rubygems'
require 'json'

The preceding code handles the former case, while the following code handles the latter:

require 'rubygems'
require 'json/pure'

The gem...