* Chemical Change: The sourness in milk is caused by the conversion of lactose (milk sugar) into lactic acid by bacteria. This is a chemical reaction where the composition of the milk changes.

* Physical Change: A physical change alters the form or appearance of a substance but not its chemical composition. Examples include melting ice or cutting paper. These changes are reversible.

Key takeaway: The change in milk's taste and smell is a result of a new substance (lactic acid) being formed, indicating a chemical change.