My goals were to get kids to talk about math, use correct math vocabulary, minimize "I don't know what to say" and possibly learn from their classmates. I think the activity accomplished all of these goals in some way.
To perform the activity, use any words or concepts that are new to the students. For my geometry class we used points, lines, planes, segments, rays, collinear, coplanar, non-collinear, non-coplanar, parallel lines, parallel planes, skew, opposite rays. Students had learned these concepts over several days, taken some notes, created paper prisms and labeled with points, then identified parallel planes, collinear points, skew lines, etc. So the students had used the words in some fashion. Now it was time to review. Students were given half index cards to make flash cards for all of the words. The front contained the word or phrase, the back was a description, drawing, or other key words that described the concept. It took students about 15 minutes to create them. Then I acted out the activity with the students by borrowing a set of someone's cards. I read the description on the back to the class and had them guess the words. For example, I said "these are lines that don't intersect and they are in different planes." Some students said parallel lines, but (and it is important to note this) many students didn't say anything at all. (Here's where you collect data. Note how many students answered aloud when you asked the class, then compare to the number of students talking when you start the activity. Any increase in the number of students engaged is success!)
Explain that students will be paired up, then each will read the descriptions to their partner and their partner must guess the word or phrase. One of my favorite ways to pair kids up is to distribute a playing card to each student. Then tell them they must get up and find the person with the same number and color. (So a red 2 finds another red 2, a black four finds another black four, etc.) Tell them they have 2 minutes to find their partner and start quizzing each other. Then (and this is important) set a timer for 2 minutes! Walk around the room and listen. First recognize that more students are talking about math than before...so yay it's working! After two minutes, tell students to go find someone with the same number but a different color and set the timer for another two minutes. Next pairing can be an even numbers must find even numbers with same color (odds must find odd numbers with same color). Then switch to even number and opposite color, etc. Depending on the number and difficulty of concepts, I try to do about 4 pairings. If you do too few, they don't get comfortable. If you do too many, they stop talking about math. Because I am circulating and listening in, I often see 100% of kids talking about math. Sometimes I listen and write down words or phrases I hear and share them with the class after the activity. I tell them these are my favorite things. I always include an error from someone and tell them how happy I was to hear that because it allowed the pair of students to have a deeper discussion about math.
Here's why I think this activity gets them talking about math. They have the vocabulary in front of them. They can take turns reading or answering. They only have to say it in front of one other person. Listening helps them with their pronunciation (non-collinear is a mouthful for non-verbal kids). They know the activity will end. (The timer is important.) They believe they can be successful before they start.
Here's my new goal. How do I create all those components in every activity I do? I think these are the key parts to getting kids to talk about math.