1. Copy one handout per pair of students, and cut the handouts up into A and B.
2. Put the students into pairs with an A and B student, and distribute the handouts.
3. Focus students' attention on the instructions and make sure they aren't looking at each other's sheets.
4. Explain to students they must read their short passages to their partner, who each time responds with the appropriate phrase using a past modal verb of deduction (must have, may have, can't have + past participle). Encourage students not to give away the answers and instead until they reach the correct answer.
5. Monitor the students, providing language input or error correction when necessary.
6. Finally, draw to their attention any appropriate vocabulary or serious language mistakes.