How it works
Sam had eight coins and a free afternoon. A boat kit cost eight coins, art pens six and a sticker book two.
The coins would buy the boat kit, or the pens and the book, but not all three.
Sam picked the boat kit. What was given up is not everything at the fair, only the next best choice: the pens and the book. That is the opportunity cost.
Time has a cost too. The afternoon on the boat meant no football with friends. Ana picked the pens and the book, so for her the boat kit was the cost.