I've been teaching economics for three decades. I ask 18-year olds the following questions. If government spending irrefutably increases our GDP, what's the logical conclusion from a policy standpoint? Using what we've learned about opportunity costs, what is a reasonable question to ask someone who asserts more government spending is the solution to our problems? They get it. When teaching production possibility analysis, I teach students (the fact!) that many politicians defend the government spending they support as "an investment in our future.." I then ask students: what;s a reasonable question to ask as a retort? They get it. They know the "bridge to nowhere" and "midnight basketball." Public choice econ, anyone?