Ruby's IRB is a pretty good environment to quickly test a snippet of code. And when you want to test a snippet of code, you do not necessarily know all of the methods and properties for the objects you are working with. If only there were a tab code-complete feature in IRB.
...which has been available in IRB since too-long-ago-to-remember.
What you neeed to do is run this one-line require and then you get autocomplete:
For instance, type
and then press Tab. Auto-complete! :-)