Modern smartphone are equipped with excellent cameras and are found in the pocket of most undergraduate and high school students. We found it useful to couple a smartphone camera with a single addition ball lens, available from several manufacturers for $40 or less, to build a simple microscope. A 3D-printed mount holds the ball lens and allows for microscope slides to be inserted and translated. When combined with back illumination (e.g. an overhead projector or flashlight), the smartphone can then be set down on the 3D-printed mount, and images or video of microscopic objects can be acquired.