12. The machine-readable storage device of claim 8 , wherein said operation of outputting at least part of the text with the indication of the corresponding explanation of each corresponding selected expression comprises: displaying the at least part of the text with the explanation of each expression proximate to each occurrence of the expression.
8. A machine-readable storage device having stored therein a plurality of instructions for facilitating computer-assisted comprehension of texts, the plurality of instructions executable by a machine, wherein said plurality of instructions when executed cause said machine to perform operations comprising: providing a digital representation of a text, the text including at least one occurrence of each one of a plurality of expressions, associating each expression with a difficulty index indicative of a corresponding comprehension difficulty, selecting a set of the expressions according to a comparison of the corresponding difficulty indexes with a threshold value, associating each selected expression with a corresponding explanation, and outputting at least part of the text with an indication of the explanation of each corresponding selected expression, 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.