If you've ever noticed tiny worm-like shapes or transparent blobs in your field of vision - you know, those things that mysteriously disappear as soon as you try to focus in on them, then reappear right when you give up and start looking at something else - don't worry, you're not crazy, and you don't have parasites in your eyeballs.
What you have is floaters, and while that name invokes certain other things we won't mention, there's nothing gross about them.
(Just FYI if you get the other kind of floaters in there... hello, pinkeye.)
Technically known as muscae volitantes, which in Latin translates to "flying flies", these annoying optical illusions are created inside your eyeball under very specific circumstances.
As the TED-Ed video above explains, floaters are tiny objects that get inside your eyeball and cast shadows on your retina - the light-sensitive tissue at the back of your eye.
They're not foreign materials, but bits of your own body that break loose - such as tiny portions of tissue, red blood cells, or protein - and end up in the gel-like vitreous humour that fills the space between the lens and the retina.
Because they're controlled by the movements of this viscous liquid, they'll move wherever your eyeball moves, and might even 'bump' against the sides as you flick your field of vision from side-to-side, trying to catch them.
If you want to get a better look at your floaters, try to find a uniform background to view them against, like a blank computer screen or a clear sky. You'll probably still struggle to catch them in focus, but you've got a much better chance of getting to know them a little better.
If you've never experienced floaters before, but have seen tiny flashes of light when looking up at the sky, you've experienced a similar - but entirely unrelated - optical illusion known as the blue field entoptic phenomenon.
I'll let the video below explain how those are formed, just don't be freaked out when you notice that these tiny flashes are blinking in and out at the exact same time as your heartbeat. Just as Conspiracy Keanu would say, everything is connected.
A version of this article was first posted in May 2016.