Although he is incapable of asking on his own initiative, we are not instructed merely to tell him what he does not know how to ask for. Rather, we are instructed to “open for him”; we are to stimulate and encourage him to ask his own question, whatever it may be, which we will use as an introduction for teaching him about the departure from Egypt. We are not to carry him into the room of knowledge; we are merely to open for him the door so that he can walk into it himself.

haggadah Section: -- Four Children
Source: The Malbim Haggadah