Everyone can suck on a straw and breathe at the same time, which can be checked by doing this: inhale as much air as you can, then a little more, until there's really no more room for more. Now suck the straw. It will still work. This is because sucking and inhaling don't use the same parts of the body, even though the result is more or less the same.
Most non-musicians, including me at first, assume you suck by inhaling, without giving this a second thought. Even when I did give it a second thought, I figured the brain was just really good at stopping the sucking when there was enough fluid in the mouth, so you wouldn't drown while drinking your lemonade. Even when I try to mimic the sucking when not actually sucking a straw, I find myself inhaling. But when you suck a straw, things are different.
When you suck a straw, your tongue starts pretty much in front of your mouth, filling up most of the free space in your mouth, and closing off the connection to your throat. Then, while your lips are sealed around the straw, your tongue moves backwards, creating more space in your mouth, while keeping the connection to your throat closed off. This creates a low pressure, which allows the pressure outside to push the fluid into your mouth. Your lungs use the same principle, except now you're not using your lungs, but rather your mouth.
Musicians have known this all along, and call it circular breathing. They use it to keep playing long notes while taking in short breaths so they can keep going. Using only your mouth effectively leaves the passageway through your nose open, which you can use to breathe, whether it be inhaling or exhaling. This isn't something you have to learn, either. You're already capable of doing it. Sucking is one of the things a sibling can already do when he's born, because it's vital to his survival. You never learned it after you were born, that's why everyone does it the same.
Since no one learns this, it's hardly ever thought about. This would explain while so many people are unaware of this, especially when you have nothing to do with brass instruments. As for me, I hadn't even heard of the term circular breathing before I wrote this.
Things to note:
- When the 'straw' gets too big, About the size where your mouth is fully opened, it's impossible for your tongue to close off your throat, while still getting a decent amount of low pressure. Then, you probably will resort to inhaling.
- When you're drinking from the stream of water out of a tap, you suck with your mouth too.
- You can actually push more air out of your mouth by blowing this way, then you can get in by sucking. This is because before you blow, you can fill your mouth with air enough for your cheeks to expand. But when you suck, you can't make your cheeks expand because humans don't have any muscles for that.