1. If the Lord doesn't build a house, the work of its builders is useless. If the Lord doesn't watch over a city, it's useless for those on guard duty to stand watch over it.
2. It's useless for you to work from early morning until late at night just to get food to eat. God provides for those he loves even while they sleep.
3. Children are a gift from the Lord. They are a reward from him.