This Instructional Activity is a part of the Lesson 20, Unit 5, Grade 8. In this activity, students begin by looking at an image of a sphere in a cylinder. The sphere and cylinder have the same radius and the height of the cylinder is equal to the diameter of the sphere. Students consider the image and reason about how the volumes of the two figures compare to get a closer estimate of the volume of the sphere. Then students watch a video that shows a sphere inside a cylinder set up like the image. A cone with the same base and height as the cylinder is introduced and its contents poured into the sphere, completely filling the empty space between the sphere and the cylinder.