My teacher told me this story. In Vedic thought, creation goes through four epochs or yugas - the Sata, Treta, Duapar, and Kal yuga, with the world now said to be in the last, Kalyuga, or the age of untruth. Lord Krishna lived through the transition from Duapar to Kalyuga.
Some devotes, including the famous Pandavas, want to understand the transition, and Lord Krishna gave many illustrations. In one instance, he asked the Pandavas to tell him what they see as they sit in meditation. The brothers see strange happenings as they close their eyes in prayer, but there was a recurrent theme that they all experienced. They saw a tall mountain, with slopes full of verdant green forests. Suddenly, a boulder starts rolling down the mountain. As it rolls, it strikes a tree, but instead of stopping, it becomes bigger in size, and knocks down the tree. This cycle of increase in size and knocking down trees continues, and the verdant mountain is being ravaged. And then, the now giant boulder strikes a gentle blade of grass. It stops, and shatters. The whole forest could not stop the destruction, but one blade of grass could.
Lord Krishna explains the vision. The boulder is ego and arrogance, which was held in check by the evergreen forest of charity and meditation. This worked in Duapar, when the devotee believed that charity and meditation were His blessings. As Kalyuga takes hold, the devotee now believes that he is in meditation, and he does charity. The more the devotee meditates and the more charitable he is, the bigger the size of the ego boulder.
Charity, fasts, pilgrimage—create ripples of Ego.//These acts are then wasted—like an elephant bathing with mud.
Until the boulder faces the gentle blade of His name and His love - and it disintegrates. Lord Krishna teaches that in Kalayuga, the easy method of self-realisation is through love. Guru Granth Sahib says:
Kalyuga is here—//Sow the seeds of love for His name.