Tell a Primary Source From a Secondary One

A textbook about World War Two is not the same thing as a soldier's diary from 1943. This explains what makes a source primary or secondary, uses three concrete examples from the field you care about, and clarifies when each type is actually more useful rather than pretending one is always better than the other.

Explain the difference between primary and secondary sources in research, using three concrete examples drawn from [one of these fields: history / science / journalism / literature]. For each example:
1. What the primary source is and why
2. What the secondary source is and why
3. What you can learn from each that you cannot learn from the other

Then explain: when should a researcher prefer a primary source, when is a secondary source actually more useful, and what a tertiary source is and when it matters. Keep everything in plain English.