Error installing rails, how to solve it?

Chanteki-MacBook-Pro:~ chanwaihung$ sudo gem install -y rails
INFO:  `gem install -y` is now default and will be removed
INFO:  use --ignore-dependencies to install only the gems you list
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-1.7.3 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/json-1.7.3/ext/json/ext/parser/gem_make.out
Chanteki-MacBook-Pro:~ chanwaihung$ rails
Rails is not currently installed on this system. To get the latest version, simply type:

    $ sudo gem install rails

You can then rerun your "rails" command.
Chanteki-MacBook-Pro:~ chanwaihung$ sudo gem install rails
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-1.7.3 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/json-1.7.3/ext/json/ext/parser/gem_make.out
Chanteki-MacBook-Pro:~ chanwaihung$ rails
Rails is not currently installed on this system. To get the latest version, simply type:

    $ sudo gem install rails

You can then rerun your "rails" command.
Chanteki-MacBook-Pro:~ chanwaihung$

try instant rails if u just need to play around with rails.

http://rubyforge.org/projects/instantrails/