Washburn's equation is a useful tool for studying the penetration of liquids in porous media because it predicts a relationship between the distance penetrated by the liquid in the porous media, x, and the time spent in this process, t. Despite its extended applicability, some problems in the interpretation of the results obtained arise from its use in the linearized form of x(2) vs t. In this paper a careful mathematical analysis of Washburn's equation shows that most of the discordance found from its application comes from inadequate use of Washburn's equation. To confirm this assessment, the penetration of different liquids in plates used in thin-layer chromatography was investigated. From that study it has been proved that the alien behavior appearing when Washburn's equation is used in a linearized form disappears when it is used in its polynomial expression.