Think that you're country A, wanting to buy pen and paper.
Country B produces 1 million pen and 1 BILLION paper
Country C produces 1 BILLION pen and 1 million paper
Or, country B has the absolute advantage over production of
paper while
country C has the absolute advantage over production of pen.
Coming back our theory of economy of scale, we know that to a
certain point, increased production would lead to lower average
cost and thus, lower price.
This would mean that paper from country B is cheaper than
country C whereas pen from country C is cheaper than country B.
Therefore, you would choose to trade paper with country B while
trading pen with country C.
And this is why it is important to making economic choices.