You cannot hum for more than a second if you hold your nose tightly shut.

To make a humming sound, your voice box needs a steady stream of moving air to make your vocal cords vibrate. Since your mouth is closed during a hum, all that air must escape through your nose. When you pinch your nostrils, you turn your throat and nose into a sealed container.This follows a rule of physics called Boyle's Law. As your lungs try to push air up, the pressure inside your throat builds up very fast because the air is trapped. Within less than a second, the pressure inside your throat matches the pressure from your lungs, and the air stops moving entirely.Without moving air, your vocal cords cannot wiggle or vibrate to create sound. In a 2012 study using acoustic analysis, researchers found that people could only hum for about 0.8 seconds after their nose was blocked. The tiny bit of sound you hear at the very start is just the last little bit of air being squeezed into the space behind your nose before it fills up completely.