This paper proposes a method for seamless interaction between students and their professor using Twitter, one of the typical social network service (SNS) platforms, in large lectures. During the lecture, the professor poses surprise questions in the form of a quiz on an overhead screen at unexpected moments, and students submit their answers through a smartphone response system using Twitter. Since points are awarded on a first-come-first-served basis to only a limited number of the students who submit correct answers, students are required to concentrate on the lecture if they want to score. The proposed method solves both the problem of decreased concentration arising from frequent use of smartphones for nonlearning purposes in lectures and of inefficient interaction that occurs in large university classrooms. Evaluation through analysis of exam results and a post-course survey confirm that the proposed solution produces better results in terms of both students' academic achievement and their concentration during lectures.