Here's the whole idea in one breath: your brain is constantly working out how far away things are, automatically, without asking you. A flat picture can feed it the exact same clues the real world uses — and the brain happily "sees" depth that simply isn't there. Artists have been hijacking that reflex for centuries.
Start with the Machine in Your Head
You almost never have to think "that car is further away than that letterbox" — you just know. That's because judging distance kept our ancestors alive: how far is that cliff edge, can I reach that fruit, is that predator close enough to worry about? So we evolved a fast, silent distance-reading machine that runs every second your eyes are open. It reads a handful of clues off the world and turns them into a sense of near and far, instantly, below the level of thinking.
The sneaky bit is this: that machine doesn't actually check whether it's looking at the real world or at a flat picture. It just reads the clues. So if a drawing serves up the same clues — something blocking something else, one shape smaller than another, a thing sitting higher up the page — the machine does its job and reports back "depth", even though you're staring at flat ink. That's not you being gullible. It's a brilliant survival system being politely tricked.
Watch the Trick Happen
In the toy you start with two plain squares, side by side, dead equal — clearly just two flat shapes on a page. Now press the toggle. Square B doesn't travel anywhere; it's the same ink it always was. But it shrinks a little, slides higher up toward the horizon, and square A shifts across so it overlaps B's corner. Suddenly you can't not see B as sitting behind A, further back in a space that doesn't exist. Three small changes, and your distance machine has been fed and fooled.
That's the foundation everything else in this concept rests on. Depth isn't something you add to a page like paint — it's something you trigger in the viewer's head by giving them the right clues. Get good at the clues and you can build a whole world that someone's brain will walk straight into.