So what are those floating things that you can see against the sky or a computer screen?

Inside your eyeball is a jelly-like material. As we get older, the material starts to dissolve.

Sometimes, some of the material doesn't dissolve and ends up floating around in the rest of the jelly and it can look like spots or wriggly lines or cobwebs. 

Mostly, these can be harmless - although sometimes distracting or annoying.

But they can be more serious. If they appear suddenly, there's a lot of them, there's a large one, there's a flashing light - you should call us to be seen on the same day. It could be a retinal detachment. These need treating straight away.

Otherwise, if they aren't serious, your brain can get used to the floaters and you may only see them against bright backgrounds.