Students' opportunities to learn science are shaped by the intellectual work in which they engage in science classrooms. By considering the opportunity to learn as a more nuanced and complex concept than simply as exposure to the subject matter, we argue that the kind of tasks that teachers assign to students presents an important element to understand how students are positioned to learn in science classrooms. Teachers, undoubtedly, play a critical role in the selection of these instructional tasks. This study aims to investigate the cognitive demand of science tasks and teachers' reasoning for what makes these tasks cognitively demanding. Guided by a framework, which was designed to classify science tasks according to cognitive demand and the integration of science content and practices, we analyzed 224 science tasks shared by 125 teachers through a statewide survey. The analyses revealed many of the science tasks, which were identified by teachers as demanding high-level intellectual work from students and were classified into low-level categories of this framework. The qualitative analyses of teachers' responses to survey questions revealed the factors that influenced science teachers' decisions about the cognitive demand of instructional tasks.