Why is the production possibilities frontier bowed out?

What will be an ideal response?

The production possibilities frontier is bowed out because resources are not equally productive in all uses. The resources used to produce robots are different from the resources used to produce pizzas. Thus, as more of one good is produced, say robots, less productive resources must be used to increase the number of robots produced. Hence the opportunity cost of the additional robots increases, which gives the production possibilities frontier a bowed out shape.