And if you've ever poked a finger deep inside your belly button, or had someone do it to you (rude!), you might have felt a weird sensation in your groin.
Specifically, a tingle in your urinary tract area.
So...why does this happen? Let's ask a doctor.
"At the navel, you have the ability to stimulate not only the skin overlying the navel, but also the fibers of the inner lining of your abdomen," Dr. Christopher Hollingsworth of NYC Surgical Associates told BuzzFeed.
"So, as you stick your finger into your belly button, it sends a signal from the deeper fibers that line your inner abdominal cavity to your spinal cord," Dr. Hollingsworth says. "Because your spinal cord at that level is also relaying signals from your bladder and urethra, it feels almost the same. You interpret this as discomfort in your bladder."
"The internal lining of the abdominal cavity at your umbilicus (belly button) is called your parietal peritoneum. This structure is exquisitely sensitive and its sensory nerve fibers relay input back to the spinal cord at the same level as the nerves that relay sensation from your bladder and urethra."
So TL;DR: When you dig inside your belly button, you stimulate abdominal tissue that's near the part of your spine that tells your brain you have to pee.