". ▁Alex ▁also ▁showed ▁some ▁compreh ension ▁of ▁personal ▁pron oun s ; ▁he ▁used ▁different ▁language ▁when ▁referring ▁to ▁himself ▁or ▁others , ▁indicating ▁a ▁concept ▁of ▁" I " ▁and ▁" you ". ▁ ▁In ▁July ▁ 2 0 0 5 , ▁Pe pper berg ▁reported ▁that ▁Alex ▁understood ▁the ▁concept ▁of ▁zero . ▁If ▁asked ▁the ▁difference ▁between ▁two ▁objects , ▁he ▁also ▁answered ▁that ; ▁but ▁if ▁there ▁was ▁no ▁difference ▁between ▁the ▁objects , ▁he ▁said ▁" none ", ▁which ▁meant ▁that ▁he ▁understood ▁the ▁concept ▁of ▁nothing ▁or ▁zero . ▁In ▁July ▁ 2 0 0 6 , ▁Pe pper berg ▁discovered ▁that ▁Alex ' s ▁per ception ▁of ▁optical ▁ill us ions ▁was ▁similar ▁to ▁human ▁per ception . ▁ ▁Pe pper berg ▁was ▁training ▁Alex ▁to ▁recognize ▁English ▁graph emes , ▁in ▁the ▁hope ▁that ▁he ▁would ▁concept ually ▁relate ▁an ▁English ▁written ▁word ▁with ▁the ▁spoken ▁word . ▁He ▁could ▁identify ▁sounds ▁made ▁by ▁two - letter ▁combinations ▁such ▁as ▁SH ▁and ▁OR . ▁ ▁Death ▁ ▁Alex ' s ▁death ▁on ▁ 6 ▁September ▁ 2 0 0 7 , ▁at ▁age ▁ 3 1 , ▁came ▁as ▁a ▁surprise , ▁as ▁the ▁average ▁life ▁span ▁for ▁a ▁grey ▁par rot ▁in ▁capt ivity ▁is ▁ 4 5 ▁years . ▁His ▁last ▁words ▁(" You ▁be ▁good , ▁I ▁love ▁you . ▁See ▁you ▁tom orrow .") ▁were ▁the ▁same ▁words ▁that ▁he ▁would ▁say ▁every ▁night ▁when ▁Pe pper berg ▁left ▁the