Stare up at the sky on a generally clear day and you may notice wispy, bright spots scattering all about. If you haven’t noticed it, try next time the weather conditions allow or use the blue field above. The latter may not be good enough for everyone, but it may work for some. The good news is that either way, that should prove that the spots aren’t evidence of aliens trying to make contact — unless the aliens can communicate both through the sky and via your computer or smartphone screen. They’re also not static. Neither are they fairies nor anything else from fantasy. And they’re not hallucinations — you’re actually seeing something there.

What are you seeing? Effectively, your own white blood cells.

Our eyes have a bunch of different parts — here is a pretty good non-gross diagram — and one of them, the retina, acts as a receptor for all the light that enters. (Wikipedia describes the retina as “serv[ing] much the same function as the film in a camera,” which may help understand how it works.) Like most other body parts, our eyes need blood and the oxygen it carries. The capillaries which carry blood to and around the eye pass over the retina. And they’re very narrow — so narrow that cells need to travel in single-file through them. This means that there’s a near-constant flow of red blood cells across our retinas — “near-constant” because red blood cells make up more than 90% of the cells in our bloodstream. White blood cells and the fragment platelet cells make up the rest of the blood’s cellular markup.

Red blood cells absorb blue light, effectively casting a shadow on the retina. But our eyes and brain “fix” this problem, correcting for the colors blocked. That would be the end of the story, but for the fact that on occasion, a white blood cell comes through the capillary’s cellular parade. White blood cells let blue light through, causing a lack of shadow matching, roughly, the outline of that cell as it passes over the retina. (The capillaries are so narrow that the white blood cell, which is usually round, has to squish a bit to get through, making the gap of shadowlessness elongated.) The added light isn’t corrected for, so we’re seeing the white blood cell pass over our retina — but not actually outside our bodies. (Thankfully!)

The effect is called the “blue field entoptic phenomenon,” with “entoptic” meaning “within the eye.” As luck would have it, it’s not very hard to find an appropriately large correct shade of blue. Just look up — assuming it’s daytime and not too cloudy.

Bonus fact : If you have blue eyes, you also probably have a genetic mutation of a gene called HERC2. This isn’t a big deal, don’t worry. But assuming the two are connected, it may mean that you and everyone else with blue eyes have a common ancestor — that is, you’re all descendants of the person in whom the gene originally mutated.

From the Archives: Temporary Blindness: Another trick our eyes and brains team up to cause.

Related: A model of the eye’s anatomy. Definitely grosser than the non-gross image above. In fact, it wouldn’t be fair to label this one “non-gross.”