Tab completion in ruby irb

just found out, that in irb you can use tab completion. It works in 1.8.x and newer.
Type in
Time.now.a <tab> <tab>
and you see

ruby-1.8.6-p399 > Time.now.a
Time.now.abort_on_exception   Time.now.abort_on_exception=  Time.now.abs                  Time.now.add                  Time.now.alive?               
Time.now.all?                 Time.now.allocate             Time.now.ancestors            Time.now.any?                 Time.now.ap_name              
Time.now.ap_name=             Time.now.args                 Time.now.arity                Time.now.asctime              Time.now.assoc                
Time.now.at                   Time.now.atime                Time.now.auto_indent_mode     Time.now.auto_indent_mode=    Time.now.autoload             
Time.now.autoload?  
...