Uncle John knows pretty much everything—and if he doesn’t, he heads his massive research library, or puts one of his many associates on the case. So go ahead: In the comments below, ask Uncle John anything. (And if we answer your question sometime, we’ll send you a free book!) Today’s question comes from Renee B., who asks…

What causes bags under the eyes?

There are a lot of reasons why your eyes might look puffy in the morning. You might not have gotten enough sleep. Your eyes are tired. You have poor eating habits, particularly eating too much salt, which leads to dehydration. You might be retaining water. You may have a sinus infection. Many of these causes stretch the skin immediately underneath the eyes, particularly pulling down the eyelids.

Under-eye bags typically fade as the day goes on, which is a sign that it was retained fluids working their way out of your system. That is, if you’re young. Those bags under the eyes, like so many other things on the body, become unattractive and permanent the older we get. As far as under-eye bags are concerned, it has to do with the fat that’s part of every human face. It’s held there by muscles and ligaments. As you age, those muscles and ligaments naturally weaken, which causes everything around them to sag…including the skin around your eyes. That in turn, makes fat more visible—the fat in this case being those bags. Similarly, collagen levels in facial skin decline with age, limiting your face’s ability to literally bounce back.

Those bags can be diminished, to a degree, with a cold compress, or sleeping with an extra pillow—the elevation helps with drainage. One thing that doesn’t empty the bags: Preparation H. There’s an old urban legend that the notorious hemorrhoid cream can shrink under-eye bags the way it shrinks, uh, other things. It’s simply not true.