Here's why:

* Ketones have a carbonyl group (C=O) where the carbon atom is bonded to two other carbon atoms. This means the carbonyl group is in the "middle" of the carbon chain.

* Aldehydes, on the other hand, also have a carbonyl group, but the carbon atom is bonded to at least one hydrogen atom. This means the carbonyl group is at the "end" of the carbon chain.

Let me know if you have any other questions about functional groups!