Don’t believe it? Give it a try.
(and yes, I wish I had a camera to capture you trying it!)
Why do you think that is?
It is because when you hum, you are actually exhaling, so if both your mouth and your nose is closed, the air can’t escape.
So, although you can hum for a very brief few second or two, you will be forced to open your mouth and catch your breath.
Forward this blog post around to your friends so they can get a laugh too.