For example, Pepperberg would show Alex an object, such as a green wooden peg or a red paper triangle.

Some psychologists raise doubts about Pepperberg's research.

Pepperberg hoped that a similar system would help Alex grasp the meaning of words, not just their sounds.

Pepperberg can show Alex two objects (for example, a green square and a red square) and ask, "What's different?"

Irene Pepperberg, a researcher at Northwestern University, is discovering that a parrot can not only mimic people but also learn the meaning of words.

But Pepperberg says working with the parrot has already changed the way of looking at animals.

To teach Alex, Pepperberg used a special form of training.

Irene Pepperberg holds a round tray in front of a parrot she has named Alex.

"How many keys?" asked Pepperberg.