After you inhale helium your voice might sound higher, but the pitch (or frequency of the sound waves) is exactly the same. Your vocal cords vibrate at the same frequency because your body doesn’t adjust for the presence of a less-dense gas in your throat. What does change is the speed of sound in helium versus air—because helium has a lower molecular weight than air, the speed of sound is higher. You’ve probably heard this is because helium is less dense—that’s not technically correct, but let’s not go there.

So the speed of sound is faster, but why does that make your voice sound weird? The tone is actually identical; what’s different is the timbre. Specifically, the lower frequencies of your voice have less power, so your voice sounds squeaky—like a duck.