With increasing focus on active learning in college classrooms, many institutions of higher education are redesigning introductory laboratory classes to provide more active-learning opportunities for students and to more authentically recreate the practices of scientists. These classes are primarily taught by graduate teaching assistants (GTAs), who often lack the pedagogical training necessary to plan for and support students' intellectual engagement in rich science tasks that require deep engagement in the practices of science and the core disciplinary ideas. We believe that graduate student discussion groups can provide an opportunity to encourage and equip GTAs with pedagogical knowledge and skills to select and use cognitively demanding instructional tasks. In this article, we describe our planning and facilitation of one such meeting with a group of GTAs about the relative cognitive demands of various laboratory activities. We propose that regularly scheduled meetings of discussion groups like this can help build learning communities among GTAs. We provide strategies to support GTAs' professional development and help them think critically about the tasks they use in their classes. In particular, we highlight the importance of the cognitive demands of tasks for engaging students in active and rigorous opportunities for science learning.