A new study provides experimental evidence that wild capuchin monkeys use more than just physical cues when selecting stone tools; they also rely on visible signs that a stone has previously been used for this purpose. Researchers from the Max Planck Institute for Evolutionary Anthropology and the University of São Paulo investigated how nut-cracking capuchin monkeys choose between stones of different hardness and appearance, examining whether evidence of prior use, as indicated by percussive damage, can override the influence of a stone's intrinsic material properties.