The law of conservation of energy says that energy cannot be created or destroyed — it can only change form or move from one place to another. The total amount of energy in a closed system always stays the same. When a ball falls, potential energy becomes kinetic energy, but the total does not change. When energy seems to disappear, it has really just turned into heat or another less obvious form.
Build a simple roller coaster from foam tubing and track a marble's speed at different heights. Discuss where the energy "goes" when the marble slows at the top of a hill. Use a bouncing ball to show that each bounce is lower because energy converts to heat and sound — not because it disappears.
One of the biggest rules in all of science is that energy cannot be created or destroyed. This is called the law of conservation of energy. It means the total amount of energy in a system always stays the same, no matter what happens inside it. Energy can move around and change form, but the total amount at the end is always equal to the total amount at the start.
Think of a roller coaster at the top of the first hill. It has lots of potential energy because of its height. As it rolls downhill, that potential energy converts into kinetic energy (speed). At the bottom, most of the potential energy has become kinetic energy. As it climbs the next hill, kinetic energy changes back into potential energy, and the coaster slows down. If there were no friction or air resistance, the coaster could go on forever without losing any energy. In reality, a little energy converts to heat from friction on the tracks and air pushing against the car, which is why each hill has to be slightly lower than the one before.
When energy seems to disappear, it has not actually gone anywhere — it has just turned into a form that is harder to notice. A bouncing ball loses height with each bounce. The "lost" energy did not vanish; it was converted into heat in the ball and the floor, and sound that you hear as the bounce. If you could measure all the heat and sound produced, you would find that the total energy is exactly the same as when you dropped the ball.
This law is why the idea of a machine that runs forever without any fuel — a "perpetual motion machine" — is impossible. Every machine converts some energy into heat through friction, so it will always slow down unless new energy is added. Power plants do not create energy; they convert the chemical energy in fuel, the kinetic energy of wind or water, or the light energy from the Sun into electrical energy. Understanding that energy is never created or destroyed is one of the most powerful ideas in science — it connects every event in the universe through a single unbreakable rule.