Crack the nut

Bron:, photo by Jose Reynaldo da Fonseca
Bearded capuchins (Cebus libidinosus) use the hammer-and-anvil technique to crack hard nuts. After placing it on a stone or tree trunk, they crack a nut with another stone.
But how are monkeys able to get the round nuts to lie still, so that they can aim their blow with the stone to crack them? A team of Brazilian primate scientists began by rolling a number of palm nuts to see how to put each down such that it would be stable; they marked that side with ink. These nuts were then given to the capuchins. In 84% of the trials, the monkeys set the nuts down in the most stable orientation before cracking them with a stone. The primates clearly had insight into the best way to place the nuts.

A short video of a nut-cracking capuchin can be viewed on the sciencemag site.
Photo license: Creative Commons Attribution-Share Alike 3.0