Imagine your body is filled with pipes—big ones and tiny ones—carrying liquid (called fluid) to all the places it needs to go (from your toes to your head).

Now, when something bothers your body, like a bug bite, a sunburn, or when you have some type of disease, some of those tiny pipes can start to leak. That leak causes fluid to build up under your skin, like a little water balloon.

That is called edema.