Here's why:

* Chemical changes involve the breaking and forming of new chemical bonds, resulting in the creation of entirely new substances with different properties.

* Examples: Burning wood (wood + oxygen --> ash + carbon dioxide + water), baking a cake (flour + sugar + eggs + etc. --> cake), rusting iron (iron + oxygen --> iron oxide).

* Physical changes only alter the appearance or state of matter, not the chemical composition. The substances remain the same.

* Examples: Melting ice (water in solid form --> water in liquid form), cutting paper (changing the size and shape of the paper, but not its chemical makeup).

In summary: Changes that involve the formation of new substances are chemical changes.