For years, when I wanted to create a drawing in Adobe Photoshop, I used the mouse. I have since traded my mouse for a tablet. However, understanding how to draw with the mouse is important for any Photoshop user. More importantly, understanding the thought process behind drawing with the mouse will give you insight on how Photoshop works. It will help you better understand how to work with layers and help you when you need to problem solve along the way. Photoshop is, after all, a program that you must problem solve as you work.
In this demonstration, I show you how to draw a sphere in Adobe Photoshop using only a mouse. It may seem like a taunting task, but actually it's not really hard. Just make sure to use the tools to their potential. Use the brush tool like an airbrush, with a check on the opacity (around 25%) and the flow (around 30%).
A larger brush typically works better for this drawing. Also be sure to keep the circular area selected when you paint the highlights and the shadows on the sphere. It's actually rather easy to draw with the mouse.