References: What is Economics?

1. Economics - Wikipedia - Comprehensive overview of economics as a social science, covering its history, major branches, and core principles including scarcity and choice.

2. Scarcity - Wikipedia - Explains the fundamental economic problem of limited resources versus unlimited wants, the starting point for all economic reasoning.

3. Opportunity Cost - Wikipedia - Covers the concept of trade-offs and the true cost of any decision, measured by the value of the next best alternative foregone.

4. Economics (7th Edition) - N. Gregory Mankiw - Cengage Learning - The gold standard introductory textbook with excellent coverage of the ten principles of economics, including scarcity and marginal analysis.

5. The Economy - The CORE Team - Oxford University Press - A free, modern introductory economics textbook that presents economic systems and decision-making with real-world data and interactive tools.

6. Opportunity Cost - Concise Encyclopedia of Economics - Clear explanation of opportunity cost with practical examples showing how economists think about the true cost of choices.

7. Marginalism - Concise Encyclopedia of Economics - Explains marginal analysis and why economists focus on the additional cost or benefit of one more unit rather than totals.

8. U.S. GDP Data - Federal Reserve Economic Data (FRED) - Real-time data on U.S. gross domestic product, useful for understanding how economists measure the size of an economy.

9. World Development Indicators - World Bank - Comprehensive collection of development data covering economic output, trade, and poverty across countries, illustrating how economists compare economic systems globally.

10. U.S. Census Bureau Economic Indicators - U.S. Census Bureau - Government source for key economic data that illustrates how economists gather and use data to understand economic activity.