The alarm clock. The personal computer. The smartphone. The radio. You know the greatest gadgets of all time (and you’ve probably owned most of them), but which has changed the world more than any other? To make our list of 101, a gadget had to be something you could hold in your hands, mechanical or electronic, and a mass-produced personal item. The rest was up to the judges. Check out our selections, and let us know in the comments what you think!