Do you know that you are working when you are playing?

To a scientist, work is any kind of action that uses energy.

Energy is needed to do all types of work—for example, to throw a ball into the air and to catch it. Energy is needed because something does not move unless you push or pull it. And it doesn’t stop moving unless something else slows it down. When you catch a ball, your hand can feel the ball pushing to continue moving. Inertia is a basic characteristic of an object as it continues to stay at rest or continues to move.
If you want to start moving something or stop it from moving, you need to push or pull it. These pushes and pulls are called forces. Forces are needed to overcome inertia. Forces are produced by applying energy. The more force applied, the more energy used and the more work done.

Energy for lifting

When you lift a heavy box, potential energy changes to kinetic energy in your muscles. You use more energy and do more work when you move a heavy box than when you lift a lighter box for the same distance. You do more work when you lift a box up to a high shelf than when you lift it onto a low shelf. If you carry a pile of books weighing 22 pounds (10 kilograms) up a flight of stairs, you do twice as much work than if you carried an 11-pound (5-kilogram) pile up the same flight of stairs. Since work is equal to force times distance, the energy you use is equal to the weight of the books times the distance you moved.