One interesting study was successfully done with chimps. Six chimps were trained to operate a special vending machine. One ripe grape came out for each while token put into the slot.

• 曾经对黑猩猩成功地进行了一项有趣的实验,实验对六只黑猩猩进行训练,让它们操纵一台专用售货机,往投币孔里塞入一枚白色筹码可得到一颗热葡萄。