REPL (Read-Eval-Print-Loop) is a great way to learn.

Sarah Allen has a great post on REPL and rspec.
Sarah’s Post on REPL Rspec

In Rspec 2.9 there is a nice way to get the standalone way working


$ irb
1.9.2p290 :001 > require 'rspec/mocks/standalone'
1.9.2p290 :002 > Time.stub(:now) {1}
1.9.2p290 :003 > Time.now
=> 1