A famous theorem discovered in 1936 by H. Steinhaus on a sufficient condition for obtaining the coordinate functions of a curve filling the unit square is revised in the present paper. Here we point out that the converse of the above theorem fails in the Lebesgue curve. A characterization of the space-filling curves by means of a filling condition is proposed. A constructive characterization of this filling condition, in terms of the Borel measures, is also settled.En este artículo revisamos un famoso teorema, descubierto por H. Steinhaus en 1936, en el que se da una condición suficiente que permite obtener las funciones coordenadas de una curva que llena el cuadrado unidad. Ponemos de manifiesto que el recíproco de este teorema no se cumpl...