If a non polar dielectric like methane is taken and placed in some external electric field. Center of positive charge of individual molecules is pulled automatically in the same direction as that of electric field towards the plate having negative charge. Similarly, the centre of the negative charged electrons is dragged in the opposite direction of the electric field, towards the plate having positive charge. So the centres of positive as well as negative charges are set apart. Due to this the molecules are deformed from their original shape and hence separate at last. So, due to the above process each molecule gets some dipole moment . After sometime these molecules will get polarized when the forces of attraction between the centres of positive and negative charges and the force due to electric field will reach at some stable state.