I love it when animals can solve puzzles and problems that I can't even figure out. Here's a crow going through a bunch of different exercises to show its understanding of size, weight, density, the elements and even the amount of effort it should put in to a puzzle to win its reward.
The crow knows that putting rocks in water will make the water rise while dumping rocks in sand does nothing. It also knows that heavier rocks do a better job of lifting the water level than lighter rocks or hollow rocks and even knows how to attack narrow tubes and wide tubes. Crows are bloody geniuses!