$ rvm use system
<i> Now using system ruby. </i>
$ irb --prompt simple
>> RUBY_DESCRIPTION
=> "ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0]"
>> h = Hash.new([])
=> {}
>> [h.default, h[0], h[1]].map(&:object_id)
=> [2156963800, 2156963800, 2156963800]

$ rvm use 1.9.1-head
<i> Now using ruby 1.9.1 head </i>
$ irb --prompt simple
>> RUBY_DESCRIPTION
=> "ruby 1.9.2dev (2009-12-11 trunk 26067) [x86_64-darwin10.2.0]"
ruby-1.9.1-head > h = Hash.new([])
=> {}
>> [h.default, h[0], h[1]].map(&:object_id)
=> [2151892720, 2151892720, 2151892720]