One of Aesop's fables may have been based on fact, scientists report. In the tale, written more than 2,000 years ago, a crow uses stones to raise the water level in a pitcher so it can reach the water and quench its thirst - demonstrating that necessity is the mother of invention. Now a study published in Current Biology reveals that rooks, a relative of the crow, do just the same when presented with a similar situation. This rook will even choose larger stones over smaller ones to speed up the task of raising the water and getting at its tasty treat.