Crow and Pitcher
A Crow, half-dead with thirst, came upon a Pitcher which had once been full of water; but when the Crow put its beak into the mouth of the Pitcher he found that only very little water was left in it, and he simply couldn‘t reach far enough down to get it. He tried, and he tried. He tried everything to push his head into the pitcher, but at last he gave up in despair. Suddenly, a thought came to him. He took a pebble and dropped it into the Pitcher. Then he took another pebble and dropped it into the Pitcher. Then he took yet another pebble and dropped that into the Pitcher, and so on.
At last, he saw the water mount up near him, and after casting in a few more pebbles he was able to drink and quench his thirst.
Little by little does the trick