Why do opportunity cost increase as you make more and more butter and fewer guns?

As you produce more butter and fewer guns, the opportunity cost increases due to the principle of diminishing returns. Resources are not perfectly adaptable; as you shift production, the factors of production (like labor and materials) become less efficient when allocated to butter instead of guns. Thus, the more butter you produce, the greater the amount of guns you have to forgo, resulting in higher opportunity costs. This trade-off reflects the increasing sacrifice of gun production for each additional unit of butter.