youcubed: Number Talks (all ages)
youcubed (Stanford University)

Number talks, developed by Ruth Parker, are an activity you can do with your kids on a walk or in a moment you are sitting together.

Inside Mathematics: Classroom Videos - Number Talks
Inside Mathematics (The University of Texas at Austin, Charles A. Dana Center)

Number talks were developed for classroom teachers to engage students in “mental math” through grappling with interesting mathematics problems.