Here's why:

* Chemical changes involve the breaking and forming of new chemical bonds. This means the atoms in the original substances are rearranged into entirely different arrangements, creating new molecules with different properties.

* Physical changes only alter the appearance or state of matter, but not the chemical composition. The same molecules are present before and after the change, just in a different form.

Examples of chemical changes:

* Burning wood: Wood reacts with oxygen, forming ash, carbon dioxide, and water.

* Rusting iron: Iron reacts with oxygen and water, forming iron oxide (rust).

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

Examples of physical changes:

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

* Crushing a can: The can's shape changes, but it's still made of the same metal.

* Dissolving sugar in water: The sugar appears to disappear, but it's still present in the water as individual sugar molecules.