Find a blank stretch of bright wall or a clear sky, let your eyes unfocus, and they will report for duty: faint gray specks, wispy threads, a translucent amoeba or two, drifting lazily across your view - a different show from the wriggling bright dots some people also see against a blue sky, which live elsewhere in the eye. Try to look directly at one and it slides away, keeping just enough distance to stay unexamined. These are floaters, and they are not on the wall or in the sky. They are inside your eye - small clumps and strands suspended in the gel that fills it, close enough to your retina to cast shadows on it. What you are seeing, in the most literal sense, is your own interior, projected onto the world. Far from being an oddity of tired eyes, they are a standard feature of the human optical system, one that ophthalmology texts describe matter-of-factly and that most people acquire more of as the years pass.
A projector problem, inverted
The setup is easier to picture backwards. Your eye works like a camera with a light-sensitive film at the back: the cornea and lens focus incoming light, and the retina records the focused image. Floating in the chamber between lens and retina is the vitreous, a transparent gel that keeps the eye's shape - clear so that light passes through unscattered. When pieces of that gel clump or condense into strands, they are still transparent in the everyday sense, but they are not perfectly uniform. Light passing a dense clump is dimmed by a whisper, and the clump throws a tiny, soft shadow onto the retina beneath it. Because the clump sits so close to the film - a millimeter or two - even a speck too small to see casts a shadow sharp enough to register, the way a hair held just above a phone's camera lens becomes a huge dark streak in every photo. The visual system, which has no way to know the shadow's origin, does what it always does: it places the percept out in the world, at whatever distance the focusing system currently favors - which is why floaters read as distant spots against the sky and nearer smudges when you look at a close wall. The eye is capable of staging several such internal shows - the wriggling bright dots against blue sky among them - but the drifting gray shadows are the most common act in that little theater, and the most patient.
Why they dance away
The dodging behavior is the signature. The vitreous is a gel suspended in a closed chamber, so when your eye rotates, the whole mass swirls along and then keeps drifting after the eye stops, like water sloshing in a carried cup. A floater therefore moves when you move your gaze, overshoots when you stop, and settles seconds later. Trying to look straight at one fails for the same reason you cannot stare at the edge of your own blind spot: fixating on the shadow means rotating the eye until the clump lies on the visual axis - and the drifting gel, trailing the rotation, rarely cooperates. The result is the familiar soft pursuit, the floater seeming to flee your gaze at the last moment. It is not fleeing; it is being dragged by the same fluid physics that move it after every blink.
Why the sky betrays them
Floaters hide indoors and appear outdoors for a reason of contrast. A shadow cast by a clump is extremely faint - it shows only against a bright, uniform field, where the retina has no texture to hide it and no edges to prioritize. A clear sky is the ideal projection screen: maximally bright, featureless, and extended. Add a bright overcast wall or a fresh snowfield and the screen improves further. This is also how floaters stay distinct from a different sky phenomenon: the tiny bright wriggling dots some people notice against blue, which come from white blood cells squeezing through fine retinal vessels - a shimmer that traces the vessels rather than drifting with the gel. And floaters are only one member of a family of experiences the eye can generate from inside: pressing gently on your closed eye produces phosphenes, colors with no light involved at all. The eyeball, it turns out, is happy to improvise scenery when the outside world fails to supply it.
Normal passengers, and the exceptions
For most people, floaters are a normal part of the eye's aging. The vitreous gradually liquefies and condenses over the decades, its clumps becoming more numerous and more noticeable; eye-health authorities describe them as a common, usually benign companion of age, with the vitreous sometimes separating from the retina in a process that suddenly multiplies floaters without threatening sight. Between those poles, ordinary floaters need no treatment and mostly need to be understood: most people stop noticing theirs for weeks at a time, because the visual system gradually discounts a steady imperfection, until a bright sky reintroduces them like an old acquaintance. The same authorities draw a careful line: a sudden shower of new floaters, flashes of light like a camera's flash at the edge of vision, or a curtain or shadow creeping across the field of view each deserve a prompt, same-day eye examination, because occasionally the retina itself needs urgent attention. That line is worth knowing and not worth worrying about - the overwhelming majority of drifting threads are exactly what they appear to be, shadows of your own vitreous, riding along in the bright weather.