The Solow model isn't useful for only thinking about entire countries: As long as the production function runs into diminishing returns and your total stock of inputs constantly wears out, then the Solow model applies. Consider a professor's knowledge of economics.

The more she learns about economics, the more she will forget (depreciation), but the more she knows, the more knowledge she can create (production).

So eventually in steady state, she will know only a fixed amount about economics, but what she knows might change over time; some decades she might know a lot about the Federal Reserve, while other decades she might know a lot about the electricity market. In any case, knowledge fades away.

a. Apply the Solow model to a chef's skill at cooking.

b. Apply the Solow model to the size of a navy's fleet of ships.

c. Apply the Solow model to the speed of a cheetah, where the input is calories.