1. A method for facilitating computer-assisted comprehension of texts by a user of a data processing system, said method comprising: accessing a digital representation of a text, the text including at least one occurrence of each one of a plurality of expressions, associating each of the plurality of expressions with a difficulty index that estimates a corresponding comprehension difficulty, determining a set of the plurality of expressions that have corresponding difficulty indexes that exceed a threshold value, associating each of the set of the plurality of expressions with a corresponding explanation, and outputting at least part of the text with an indication of the corresponding explanation of each of the set of the plurality of expressions included in the at least part of the text that is output, updating the difficulty index associated with a first of the plurality of expressions in response to detecting selection of the first of the plurality of expressions in the at least part of the text, wherein said updating represents an increase in the estimated comprehension difficulty of the first of the plurality of expressions, associating the first of the plurality of expressions with a corresponding explanation, and adding an indication of the corresponding explanation of the first of the plurality of expressions to the output of the at least part of the text.