Because of the first stage, when students simply THINK, there is Wait Time: they actually have time to think about their answers. Because it is silent thinking time, you eliminate the problem of the eager and forward students who always shout out the answer, rendering unnecessary any thinking by other students. Also, the teacher has posed the question, and she has EVERYONE thinking about the answer, which is much different from asking a question and then calling on an individual student, which leads some students to gamble they won't be the one out of 30 who gets called on and therefore they don't think much about the question. Students get to try out their answers in the private sanctuary of the pair, before having to "go public" before the rest of their classmates. Kids who would never speak up in class are at least giving an answer to SOMEONE this way. Also, they often find out that their answer, which they assumed to be stupid, was actually not stupid at all...perhaps their partner thought of the same thing. Students also discover that they rethink their answer in order to express it to someone else, and they also often elaborate on their answer or think of new ideas as the partners share. These, it seems, are powerful reasons to employ Think-Pair-Share in order to structure students' thinking and their discussion.