LESSON 3 · Forces You Feel Every Day
Nature's Accounting
In any isolated system, total energy stays exactly constant. This is the law of conservation of energy, one of the most thoroughly tested principles in all of science — every experiment on closed systems confirms it.

When you burn gasoline, chemical energy becomes heat and motion. When a plant grows, light energy becomes chemical energy. When you eat food, chemical energy becomes muscle movement and body heat. Every energy transaction is a conversion, never a creation.