The aim of this study is to analyse the effect of tasks on the detection of explanation obstacles when secondary school students read scientific texts. Students were instructed to read short passages under different task conditions, and to ask questions if necessary. Obstacle detection was operationalised in terms of the type of questions asked by the students. The experiment examined the influence of goals associated with the task of reading to understand a text vs. reading to perform a procedure described by the text (a science experiment). Significantly, more explanation obstacles were found in the understanding condition than in the experiment condition. Scientific text also had an effect on the explanation obstacles detected.