Although beloved of some chemists and physicists, science demonstrations have been criticized for stifling inquiry and assisting teachers to maintain a power differential between themselves and students in the classroom. This interpretive study reports the unexpected positive learning outcomes for urban science students in two chemistry classes that resulted from the use of science demonstrations during a unit on gas laws. Beginning with an examination of science demonstrations as sites of interactions, researchers observed greater student engagement and positive emotional energy, more sophisticated use by students of symbol systems associated with chemistry, and a greater willingness of students to move between description of the phenomena and submicroscopic explanations. Applying sociology of emotions to analysis of classroom conversations and actions, we examine the nature of engagement and propose explanations for the positive effect of science demonstrations on the engagement, emotional energy, and learning of students. (Contains 4 figures.)