Here's why:

* Physical changes only alter the appearance or form of a substance, not its chemical composition. This means the molecules themselves stay the same.

* Chemical changes result in the formation of new substances with different chemical properties. This happens because the molecules are rearranged or broken apart to form new bonds.

Examples of physical changes:

* Melting ice: Water (H₂O) changes from solid to liquid, but it's still water.

* Boiling water: Water (H₂O) changes from liquid to gas (steam), but it's still water.

* Crushing a rock: The rock is broken into smaller pieces, but it's still the same type of rock.

* Dissolving sugar in water: Sugar (C₁₂H₂₂O₁₁) is dispersed throughout the water, but it doesn't change its chemical structure.

Examples of chemical changes:

* Burning wood: Wood reacts with oxygen to produce ash, carbon dioxide, and water - completely new substances.

* Baking a cake: Flour, eggs, sugar, and other ingredients react to form a cake - a new substance.

* Rusting iron: Iron reacts with oxygen to form iron oxide (rust) - a new substance.

Remember: Physical changes are reversible (you can usually get back to the original substance), while chemical changes are often irreversible.