I recently saw a video of a [crow using water displacement to retrieve food](https://www.youtube.com/watch?v=ZerUbHmuY04). In several experiments, the crow was able to retrieve the food more efficiently by putting rocks into the narrower tube out of two available. It also chose solid weights instead of hollow ones, and discovered which of two tubes affected water levels very quickly. My question is, how impressive is this? What other animals would be intelligent enough to do this? At about what age could a human child figure this out?