Space. Index cards with questions & correct answers on the opposite sides
Students form inner and outer cirles facing each other. Partners taje turns asking each other the questions. If necessary the answer is given. After the exchange is complete, the teacher signals and one of the circles is directed to move X number of students to the right or left, and a new round begins.
Numbering off A B A B before forming circles makes this process quick. If space is an issue, form two lines and have the lines move with the end person moving to the opposite end of the line.
Spencer Kagan, Cooperative Learning: Resources For Teachers