We present, results from a two dimensional numerical simulation of a stably stratified layer heated from below. The aim of the simulation is to study the mechanism of erosion of a stably stratified medium by the convective velocity. We solve transport equations for momentum, heat and species using Patankar's SIMPLER method. Tracer particles are used to get flow pattern inside the mixed layer and in the gradient zone. We have observed randomly moving thermals issuing from the bottom heated surface and from the interface separating the mixed layer and the gradient zone. Stable fluid from the gradient zone gets entrained into the mixed layer with the falling thermal from the interface. This is the main mechanism of the turbulent entrainment. Results from the numerical simulation indicate the need to include the effects of the turbulent entrainment in modelling the equilibrium condition.