There are really only a handful of clues your brain reads to judge distance — and the good news is you can draw every one of them on purpose. Here are the five, from the bluntest to the most subtle. The trick isn't picking one; it's layering them, exactly like the toy does when you stack the switches.
1 · Overlap — the Bluntest, Strongest Clue
If one thing blocks part of another, your brain instantly decides the blocker is in front. That's it. No maths, no perspective lines — just "I can see all of this one and only part of that one, so this one's nearer." It's the single most powerful depth clue there is, which is why even a toddler's drawing reads correctly the moment one shape covers another. In the toy, flick overlap on and the front trees slide across to partly hide the ones behind. Already the flat row starts to sort itself into front and back.
2 · Relative Size — Further Means Smaller
We know roughly how big trees, people and cars usually are, so when one is drawn smaller than its mate, the brain doesn't think "tiny tree" — it thinks "further tree". Shrink the back trees and they retreat. The catch is the brain only buys it for things it expects to be the same size; draw a giant ant and you'll just get a giant ant. Same-kind objects at different sizes, though, read as the same object at different distances every time.
3 · Placement — Height on the Page
On flat ground, the further away something sits, the higher up the page it appears — climbing toward the horizon line. Your feet are near the bottom of your view; the distant hills are up near the skyline. So lifting the back trees up toward the horizon tells the brain "these are far off", even before you've touched their size. Bottom of the page reads as near; up by the horizon reads as miles away.
4 · Detail & Texture — Sharp Near, Simple Far
Close up you can see every leaf, every crack, every fibre. Far off, all that fine detail blurs into a smooth, simple shape — you just can't resolve it. So pack texture and crisp edges onto the near objects and keep the far ones plain and soft, and the eye sorts them by distance. It's why a foreground rock gets every chisel mark while the distant mountain is one calm silhouette. In the toy, the detail switch sprinkles leaf-texture onto the near trees only.
5 · Atmospheric Perspective — Far Things Go Pale and Blue
This is the most beautiful one. Between you and a distant hill sits kilometres of air, full of dust and moisture, and all that air scatters the light. So far-off things lose their punch: they go paler, bluer and lower in contrast, fading toward the colour of the sky. Painters call it atmospheric (or aerial) perspective, and it's the clue that makes a misty mountain range feel genuinely huge and far. Switch it on and the back trees wash out toward the haze.
Play the real game in the toy: turn on just one clue and notice how weak it is on its own. Then add a second, a third. By the time all five are stacked, those four identical trees aren't a flat row any more — they're a path leading away from you. That layering is the method. You're not drawing depth; you're drawing the clues, and letting the viewer's brain do the rest.