1. Present the physical scene
Hold your thumb up close, a hand's length from your nose. Look at it. It is sharp. Now, without moving your thumb, look through it at the far wall or out the window. The wall snaps into focus — and your thumb goes blurry.
Look back at the thumb. It sharpens. The wall blurs.
You just did this with no buttons, no knobs, no effort you could feel. Near, far, near, far — clear every time, in less than a second.
Here is the idea we are chasing all lesson: something inside your eye is changing, every time you shift from near to far. It is not your eyeball moving. It is not the screen at the back sliding around. One part is quietly reshaping itself. Finding that part, and saying exactly what it does, is the whole job.
Stop scrolling. Try it in your head before reading on. When you switched from the thumb to the far wall, did anything about your eye feel different? Most people feel nothing — and that is the puzzle.
You can now state the puzzle: the same eye focuses a near object and a far object on the same fixed screen, with no part you can feel moving. What is changing?