I really love the Ruby naming conventions around this: `!` indicates mutation.

  2.4.1 :001 > a = [4,3,5,1,2]
   => [4, 3, 5, 1, 2]
  2.4.1 :002 > a.sort
   => [1, 2, 3, 4, 5]
  2.4.1 :003 > a
   => [4, 3, 5, 1, 2]
  2.4.1 :004 > a.sort!
   => [1, 2, 3, 4, 5]
  2.4.1 :005 > a
   => [1, 2, 3, 4, 5]