So, I had assigned dialogues that were supposed to use: simple past, past progressive, past perfect, and past perfect progressive (if you´re curious, no, I had no idea what those were before two weeks ago). So, part of the conversation went as follows:

1: How have you been?
2: I have been fine, thank-you. Why do you have broken legs? Have you been kicking balls?
3: Yes, I have.

It was hard to distiguish whether the word was bulls or balls, but both were equally intriguing, and made for good conversation in class afterwards.

By the way, I´m actually enjoying parsing the English language out into definable units... it removes much of the taking the construction of language for granted. It can be very hard to explain why we do something if you can´t quite see the patterns language holds.