▁difficult ▁tests ▁meas uring ▁whether ▁humans ▁have ▁achieved ▁Pi aget ' s ▁Sub stage ▁ 6 ▁object ▁perman ence . ▁Alex ▁showed ▁surprise ▁and ▁anger ▁when ▁confront ed ▁with ▁a ▁none x istent ▁object ▁or ▁one ▁different ▁from ▁what ▁he ▁had ▁been ▁led ▁to ▁believe ▁was ▁hidden ▁during ▁the ▁tests . ▁ ▁Alex ▁had ▁a ▁voc ab ul ary ▁of ▁over ▁ 1 0 0 ▁words , ▁but ▁was ▁exception al ▁in ▁that ▁he ▁appeared ▁to ▁have ▁understanding ▁of ▁what ▁he ▁said . ▁For ▁example , ▁when ▁Alex ▁was ▁shown ▁an ▁object ▁and ▁was ▁asked ▁about ▁its ▁shape , ▁color , ▁or ▁material , ▁he ▁could ▁label ▁it ▁correctly . ▁He ▁could ▁describe ▁a ▁key ▁as ▁a ▁key ▁no ▁matter ▁what ▁its ▁size ▁or ▁color , ▁and ▁could ▁determine ▁how ▁the ▁key ▁was ▁different ▁from ▁others . ▁Looking ▁at ▁a ▁mirror , ▁he ▁said ▁" what ▁color ", ▁and ▁learned ▁" gre y " ▁after ▁being ▁told ▁" gre y " ▁six ▁times . ▁This ▁made ▁him ▁the ▁first ▁and ▁only ▁non - human ▁animal ▁to ▁have ▁ever ▁asked ▁a ▁question — and ▁an ▁exist ential ▁question ▁at ▁that . ▁( A pes ▁who ▁have ▁been ▁trained ▁to ▁use ▁sign - language ▁have ▁so ▁far ▁failed ▁to ▁ever ▁ask ▁a ▁single ▁question .) ▁ ▁Alex ' s ▁ability ▁to ▁ask ▁questions ▁( and ▁to ▁answer ▁to ▁Pe pper berg ' s ▁questions ▁with ▁his ▁own ▁questions ) ▁is ▁documented ▁in ▁numerous ▁articles ▁and ▁inter views . ▁ ▁Alex ▁was ▁said ▁to ▁have ▁understood