The human toll and economic costs mounted rapidly after the United States and Israel attacked Iran on Feb. 28.

© Tierney L. Cross/The New York Times

The company’s blockbuster trading debut is the latest example of how individual investors have become a force in financial markets.

© Andres Kudacki for The New York Times

SpaceX has rapidly become one of the world’s most valuable companies. Even so, experts say investing in single stocks can be risky.

© Connor Robinson