Comma (,) generates a newline in Ruby "puts"

ruby-1.9.2-p290 :010 > first = "ruby"
=> "ruby"
ruby-1.9.2-p290 :011 > second = "rails"
=> "rails"
ruby-1.9.2-p290 :012 > puts first + second
rubyrails
=> nil
ruby-1.9.2-p290 :013 > puts first, second
ruby
rails
=> nil