Although a chewed piece of gum can be thought of as gross or useless, drawing it can be a great exercise in using value to create subtle dimension.
Step 1: Start with an organic shape for the outline of the gum.
Step 2: Add some curved lines at the top and bottom with some tight scribbles in the middle as shown. Notice the direction of the curved lines to replicate a bite mark.
Step 3: Add an even layer of tone inside the whole shape. Press slightly harder under the top curves and above the bottom curves to show an indent. Widen the center scribbles a bit.
Step 4: Smooth tones together with a blending tool. Redo step 3 after blending to intensify tones. Add an organic shaped shadow around the curves of the left and bottom for a shadow.
Step 5: Darken some of the curves to create contrast. Use a kneaded eraser to remove some pigment from the tops of the curves as shown for highlights.
