How To Make An Element Animate/Move Back and Forth (ON ITS OWN)

How To Make An Element Animate/Move Back and Forth (ON ITS OWN)

I am trying to make a DIV move up and down on the page, by itself. How do I do this? I want it to move as soon as the page loads, without pressing or clicking anything. And I want it to go in a "loop", so it will go back and forth constantly ( side to side/ up and down).