Jamieson, Fausset and Brown on Matthew 21:27

27. And they answered Jesus, and said, We cannot tell--Evidently their difficulty was, how to answer, so as neither to shake their determination to reject the claims of Christ nor damage their reputation with the people. For the truth itself they cared nothing whatever.

Neither tell I you by what authority I do these things--What composure and dignity of wisdom does our Lord here display, as He turns their question upon themselves, and, while revealing His knowledge of their hypocrisy, closes their mouths! Taking advantage of the surprise, silence, and awe produced by this reply, our Lord followed it up immediately by the two following parables.

Parable of the Two Sons (Mt 21:28-32).