The ability to automatically print on both sides of the paper not only saves time; it also saves money by using up to 50% less paper.
A two-sided printing accessory, called a duplexer, is an easy add-on to most HP inkjet printers (including All-in-Ones).
You can always print double sided the old-fashioned way too. Simply remove the page you just printed and place it back into your paper tray.
NOTE: Different printers load paper differently. You may have to print a test page, mark it, reload it, and print again to ensure that your second page isn't printing on the same side as your first printout.