How do we realise we are mortal ? The question is focussed on realising.If it is otherwise,consciousness of mortality is merely a socially acquired knowledge or is based upon own observation but that is not realisation.Strictly speaking one realises one is mortal only at the time of death and unfortunately the moment you are dead you can no longer realise anything . Talking of realising ,one always carries the feeling of transience all the while as though it is a small little umbrella which comes with you wherever you go in rain. The umbrella protects you from the harshness of the realities because if you are aware of the impermanence inherent in the scheme of things ,you do not take a stand in life- a fixity of purpose which is not warranted. If you are a mere bird of passage , of what use is a stance ,a fixity of mind ?