This Instructional Activity is a part of the Lesson 11, Unit 5, Grade 8. Show students pictures of the spheres shown below (or use real objects). Ask the class to come up with a mathematical definition on their own. Finally, we come to the volume of a sphere of radius r. First recall that a sphere of radius r is the set of all the points in three-dimensional space of distance r from a fixed point, called the center of the sphere. So a sphere is, by definition, a surface, or a two-dimensional object.