Other
Aaii: Investing Basics
Learn the basics of successful investing. Covers risk and return trade-off, diversification, asset allocation, and more.
Math Is Fun
Math Is Fun: Investing (Introduction)
An introduction to investing. Touches on basic topics - how to reduce risk, comparing investments, and the importance of diversification.
Federal Reserve Bank
Federal Reserve Bank of Atlanta: Saving and Investing
Explains the Rule of 72 with examples of compound interest and its power to help grow students' money.
Other
Fidelity: Why Diversification Matterns
This is a very thorough explanation of the importance of diversifying investments and how to go about making those plans.