Why does oily food leave a translucent patch on paper?

Oily food leaves a translucent patch on paper because fats and oils soak into the paper fibres and reduce the scattering of light. This makes the paper appear transparent or greasy, even after it has dried.

Unlike water, which evaporates completely, fats do not evaporate at room temperature. The permanent translucent spot indicates the presence of lipids in the food. This principle is used in the grease spot test, a simple method for detecting fats in school science experiments.