Krishna says "Every human nature and the deeds he does are different from person to person though both the bodies are made up of the same five basic elements. The three Guna's are the reason because of which every human behaviour, his Karma & his deeds are different from one another. Out of these Guna's first is 'Tapas', desire for darkness, those who do not think about pros & cons before doing their karma fall in this category. Like animals & birds fall under this category since they only seek bodily pleasures and do not think before an act........"