That is the question of this lesson. Some nouns use o possession, and some use a possession. There are some general rules to keep in mind but at times there are some iffy words.
You use the o version for clothes, body parts, thoughts, the family that you have when you are born, friends, home, transportation, etc. You use the a version for things that you obtain or produce like money, food, speech, spouse and family you create like kids and grandkids, etc.