This article presents two classroom episodes in which students were exposed to the value of asking questions and to the different roles played by proof in mathematics. The conversation in the two episodes is outlined in the article. The setting was a classroom of fifteen good high-school students, who were studying calculus. These episodes occurred spontaneously, after the discussion of certain theorems during the lessons. Each one of these theorems will be labelled trigger of the episode, since they inspired students to ask the initial question that led to the whole episode. These episodes show that a classroom climate in which students formulate questions and not simply solve already given problems is an environment that may lead students to the appreciation of the nature of mathematical proofs as well as the different roles they play during a mathematical activity.