- A nest contains eggs
- Birds sit in nests
- Hence, birds sit on eggs

- Birds sit on eggs
- Sitting on something provides warmth to that something
- So, a bird sitting on an egg provides warmth to that egg

- An egg may contain a young animal
- An animal requires warmth for survival
- Hence, an egg requires warmth to survive

- A bird sitting on an egg provides warmth to that egg
- An egg requires warmth to survive
- So, a bird sits on an egg to provide it with warmth for survival

Therefore, a bird sits on an egg to provide it with warmth for survival
Q: Most birds sit on their eggs until they hatch. Which of these is the most important reason why birds sit on their eggs?
Context:
- A nest contains eggs 
- Birds sit in nests 
- Sitting on something provides warmth to that something 
- An egg may contain a young animal 
- An animal requires warmth for survival