I know the phrase "correlation doesn't equal causation" but I lose points explaining why.
Accepted answer
Name the two specific problems rather than repeating the slogan.
**Directionality** — a correlation between A and B doesn't tell you which came first. Does less sleep cause lower grades, or does struggling in school cause less sleep?
**Third variable** — some unmeasured factor may cause both. Stress could reduce sleep *and* lower grades, with no direct link between them.
Only an experiment fixes this, because random assignment plus a manipulated independent variable rules out both. In a free-response answer, apply the two problems to the actual variables in the prompt — that specificity is what earns the point.