A spinning yo-yo actually has two different kinds of kinetic energy: one kind because it's moving up and down the string and another kind because it's spinning around. When you release the yo-yo from your hand, it falls toward the ground just like a stone, and it picks up speed because it's falling. But a yo-yo is different from a stone because it has string wrapped around its axle. As it falls, it starts to spin. That's why a yo-yo falls much more slowly than a stone: some of the energy that should be making it fall quickly is actually being used to make it spin around at the same time.
Just as a yo-yo has two kinds of kinetic energy, so it has two kinds of momentum: linear momentum (because it moves in a straight line, up and down on the string) and angular momentum (because it spins around). All spinning objects have angular momentum. And anything that's spinning around likes to keep on spinning so its angular momentum stays the same. If you try to make it