When I was a kid, my parents bought me a yo-yo. At first I was puzzled and wondered how to play with it. I spent a good amount of time practicing and I could finally make it roll and then come back to me. I thought that was a huge accomplishment, but then I saw on TV a yo-yo contest with these people doing insane tricks with their yo-yo. I never knew how they could do it. So I decided to see the physics behind a yo-yo. I found that when people do string tricks that makes the yo-yo roll on the string is due to friction. There is friction between the string and the axle that prevents the yo-yo from spinning, allowing the yo-yo to roll on its own string and not giving out more string. This may seem simple but it applies to an yo-yo. Newtons first law that says that an object will stay in motion until an outside force is acted upon it. This is why a yo-yo comes back to your hand. You can flick your wrist and cause a force that push the yo-yo from its string and then you flick your wrist back to get the yo-yo back to its original state.