Take a moment to look up at the sky. Whether it’s the clear blue of a sunny day or the overcast gray of winter, you should be able to see a familiar sight—slightly darker or fuzzy blobs that dip and float when you shift your gaze and sparkling dots that zig and zag. Any bright, uniform background should work. If you’re reading this at night, try a blank computer screen.

The "small worms or transparent blobs" are eye floaters or muscae volitantes, Latin for "hovering flies," explains Michael Mauser in this TED Ed video (via CasesBlog). "They might be bits of tissue, red blood cells or clumps of protein," the narrator explains, all floating in the vitreous humor contained within your eyeball. When the floaters drift close to the back of your eye, they cast a shadow on the retina that you can see against a uniform background.

The second phenomena, "dots of light darting about," is called the blue field entoptic phenomenon because it's easiest to see against a uniform blue field. These lights are caused by white blood cells coursing through the tiny capillaries on the retina’s surface. The cells are nearly the width of those capillaries, so they cause a little bit of a traffic jam, where red blood cells pile up behind them and a clear plasma space builds up in front of them. That clear space lets light pass through and we see a quickly moving dot of white.

These are just two of the most common entoptic phenomena — the phrase for things we see that originate inside the eye itself.