I rolled my kneaded eraser into a smooth 'snake' and lightly rolled it over the puppy. This is what creates that 'hole' I spoke of, which the puppy now seems to be sitting in front of. Another technique meant to represent a third dimension. You can also see where I pushed her arms back from her legs by making them slightly darker at overlap, and the feathers become darker as they recede. You have to remember that this use of tone is separate from using tone to define an object from a single light source, although all the tehniques here can be used in conjunction.
Lighting is very, very important to an artwork.
The simplest way to light a picture is with one source. More light sources, such as ambient lighting or spotlighting, start to become very complicated, and the subject isn't needed here. On the picture of the elf below, my light source was on the upper left. You can see how the 'shadows' and tone reflects this, falling on the lower right of objects and defining the shape of them. When drawing cast shadows, don't draw them in the shape of the object which cast them, but in the shape of what they are falling on , as the shadow of the elf on the rock does.