Our Rabbis said that when Moses our teacher, peace be upon him, was tending the flock of Jethro in the wilderness, a little kid escaped from him. He ran after it until it reached a shady place... When it reached the shady place, there appeared to view a pool of water and the kid stopped to drink. When Moses approached it, he said: 'I did not know that you ran away because of thirst; you must be weary.' So he placed the kid on his shoulder and walked away. Thereupon God said: 'Because thou hast mercy in leading the flock of a mortal, thou wilt assuredly [Lit. 'by thy life.'] tend my flock Israel.' Hence, 'Now MOSES WAS KEEPING THE FLOCK.'"