Here's the whole idea in one breath: in the real world there are no outlines at all — only edges where light meets dark — and your brain reads those soft slides from bright to shadow as solid, rounded form. An outline says "here is the boundary of a shape." Shading says "here is a thing you could cup in your hand."
Start with Something You've Already Seen
Look at your own hand under a lamp. There's no black line drawn around your knuckles — there can't be, lines don't exist out here. What you actually see is the back of the hand catching the light, the sides falling into a softer grey, and the gaps between the fingers dropping into proper dark. Your eye stitches all those values together and reports back: that is a rounded, three-dimensional hand. You never needed an outline. The light did the telling.
Now draw a plain circle. With just a line around it, it stays stubbornly flat — a coin, a button, an O. But darken one side, let it fade gently toward a lit edge, and tuck a shadow underneath, and the very same circle swells into a sphere you'd swear you could pick up. Nothing about the outline changed. You only added what light does — and suddenly there's a ball where a coin used to be.
The Two Ideas That Do All the Work
Tone (artists also say value) just means how light or dark a patch is, ignoring its colour entirely. A lemon and a tennis ball can sit at the exact same tone even though one's yellow and one's green. Squint at anything and the colour drops away — what's left is pure tone, and that's the raw material of solid form.
Modelling is the act of laying those tones down so a flat mark turns into something rounded. To model a sphere is to give it the gentle slide from highlight to shadow that your brain reads as roundness. Drawing the outline is the easy half; modelling the form inside it is where a flat oval becomes a real face.
In the toy, flip between the flat outlined circle and the shaded one. Same boundary, same size, same spot on the screen. One reads as a disc; the other quietly bulges into three dimensions. That little "pop" is your own brain doing exactly what it does to your hand under the lamp — and it's the entire reason shading works.