Here come the boogers
OK, now that you know where mucus comes from and what it does, it should be no surprise to find out that some of this delightful mucus finds its way down to the tips of your nose. Mucus coats the lining of our beaks for the above reasons -- to help sift out stuff we don't want to breathe in -- but the mucus that settles near the ends of our nostrils tends to get dried out, being so much closer to the open air and all. Thus, boogers are born.
Boogers come in all forms, and while some minor dried mucus isn't anything to write home about, some snot rockets are colorful, or impressively big, or are so dried out they become impacted in your nose. Neat! If you've ever suffered from an upper respiratory infection, you've probably noticed that your snot production increases, as does your booger production, especially while you sleep. It's harmless, as long as you can still breathe, so try not to freak out too much at your body's normal functioning.
Everyone gets boogers on occasion, and while they may be an unwelcome side effect of being human and possessing a body determined to keep out contaminants, they're completely normal. But just because they're normal, it doesn't mean you need to let them flap freely, and be sure to thank your friends who let you know you've got a visible booger on board.