I am trying to install rails on Ruby 1.9.3.

When I execute the command:
$ sudo gem install rails

I receive the error below. How do I resolve this?

Building native extensions.  This could take a while...
ERROR:  Error installing rails:
  ERROR: Failed to build gem native extension.

        /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
extconf.rb
mkmf.rb can't find header files for ruby at
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h


Gem files will remain installed in
/Library/Ruby/Gems/1.8/gems/json-1.6.6 for inspection.
Results logged to
/Library/Ruby/Gems/1.8/gems/json-1.6.6/ext/json/ext/generator/gem_make.out
Benjamins-MacBook-Air:~ benrweiss$ sudo gem install rails
Password:
Building native extensions.  This could take a while...
ERROR:  Error installing rails:
  ERROR: Failed to build gem native extension.

        /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
extconf.rb
mkmf.rb can't find header files for ruby at
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h


Gem files will remain installed in
/Library/Ruby/Gems/1.8/gems/json-1.6.6 for inspection.
Results logged to
/Library/Ruby/Gems/1.8/gems/json-1.6.6/ext/json/ext/generator/gem_make.out

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