That's a great question! When two or more substances combine to form one substance, it's called a chemical reaction.

Here's why:

* Chemical change: A new substance is formed with different properties than the original substances. This means the atoms have rearranged and bonded differently.

* Irreversible (usually): While some chemical reactions can be reversed, many are irreversible. This means you can't easily separate the original substances back out.

Examples of chemical reactions:

* Burning wood: Wood (made of cellulose) reacts with oxygen in the air to produce ash, carbon dioxide, and water.

* Baking a cake: Flour, sugar, eggs, and other ingredients react together to create a new substance with a different texture and taste.

* Rusting: Iron reacts with oxygen and water to form iron oxide (rust), which is a new substance with a different color and texture.

Let me know if you have any other questions about chemical reactions!