This paper describes a software system intended to help students learn about the process of tagging, the assignment of part-of-speech labels, which is basic to computational linguistics. It does this by (a) making it transparent why words have been analysed in a particular way, and (b) making it straightforward to add items to the system's lexicon and improve its grammatical coverage. An unforeseen and welcome side-effect is that users are led to reflect on various properties of English grammar.