Go further

Subject-specific further reading.

These original lessons introduce the topic. The resources below offer additional information from educational institutions or public agencies. They did not review or endorse this site. Educators should select individual pages appropriate to learners’ ages.

Using an external source responsibly

Check the page’s author, date, purpose, and evidence. Record the exact page you use rather than citing an organization’s homepage. Separate direct observations, reported findings, and your own interpretations. External websites have their own privacy practices and usage terms.