What do these two changes have in common?
milk going sour
rust forming on a metal gate

 - Both are only physical changes.
 - Both are caused by heating.
 - Both are chemical changes.
 - Both are caused by cooling.
Step 1: Think about each change.
Milk going sour is a chemical change. The type of matter in the milk slowly changes. The new matter that is formed gives the milk its sour taste.
Rust forming on a metal gate is a chemical change. As the gate rusts, the metal turns into a different type of matter called rust. Rust is reddish-brown and falls apart easily.
Step 2: Look at each answer choice.
Both are only physical changes.
Both changes are chemical changes. They are not physical changes.
Both are chemical changes.
Both changes are chemical changes. The type of matter before and after each change is different.
Both are caused by heating.
Neither change is caused by heating.
Both are caused by cooling.
Neither change is caused by cooling.