Age and reading ability of children
Do genetic factors affect reading ability in primary children?
Use this diagram to explain that the marginal relationship between Y and X in a scatterplot may be misleading if the data are observational.
Height is moderately correlated with Reading ability.
However click Slice and drag the slider. There is little relationship within any age group — Height and Reading ability are uncorrelated conditional on age.
The marginal relationship was caused by a lurking variable, Age.
The data are artificial.