X-ray reflectivity is well established and in principle quite easy method for determining the density depth-profile of thin layers. From the observed thickness fringes one can easily deduce the layer thickness. However, sometimes the fringes are not visible and still it is possible to estimate the thickness. Sometimes the assumed model is so simple that never fits the collected data completely but the resulted density profile is more less identical with the result obtained using much more complicated model. Several examples motivated by the fight with the referees will be shown and discussed during the presentation.