Scientists define energy as the ability to do work, that is to make things move or to make new chemical compounds. Work is done when a force (a push or a pull) acts on something for a distance.
Mechanical energy is the most common type of energy that we see around us. All moving objects have a type of mechanical energy called kinetic energy. When they move, they can lose energy to friction which creates heat and noise. Or the kinetic energy can be stored up as potential energy.
Potential energy is the energy that a substance or object has because of its condition (e.g., the chemical potential energy of gasoline) or position (e.g., the gravitational potential energy of a car on a hill). Potential energy can be changed into kinetic energy, which can be changed back into potential energy. For example, a car's engine changes the chemical potential energy of gasoline into the kinetic energy of its speed along the highway. When the car coasts up a hill, it changes the kinetic energy into gravitational potential energy.