Recent research highlights the paradoxical importance of students' being able to check their understanding with teachers and of teachers' constraining student participation. Using quantitative discourse analysis, this paper examines third graders' discursive strategies in initiating such checks and teachers' strategies in constraining them. The results show students regularly raise hands outside teacher-led discussions, presumably to initiate interactions, but teachers seldom acknowledge them and, when they do, often use discursive strategies that constrain student contributions. However, although teachers ignore most unauthorized utterances, they respond favorably to called-out requests and repairs, thus enabling students to check their understanding without teachers' having to call on them outside controlled discussions. Some students seem ill disposed to call out, though, which may impede their ability to check their understanding, thus hindering their learning. Evaluation of various alternatives shows none is sufficient by itself. Offering students multiple ways of checking their understanding is recommended. (Contains 2 figures and 14 tables.)