Fluorescence is a well-known phenomenon with many practical applications that has been studied for decades. Despite this, a commonly used mathematical formalism to describe how it evolves over time does not make physical sense under certain conditions, as researchers from the Institute of Physical Chemistry, Polish Academy of Sciences have recently discovered. Namely, they showed that a mathematical tool that can be safely used in solids, where molecules are practically immobile, cannot always be used in liquids, where they can move freely. If applied incorrectly, this widely used approach can lead to erroneous interpretations of experimental data and wrong conclusions. This is what their recent paper in the Journal of Chemical Physics warns us about.