Me and Abby had so much fun doing this. I tried to make my voice squeaky with the helium gas. I couldn't get it to start with, but after quite a few attempts and feeling rather ill, I got it down. Now I will share it with you:
HOW TO MAKE YOUR VOICE SQUEAKY WITH HELIUM GAS
Suck it in and hold your breath. DON'T BREATHE IT OUT AGAIN LIKE I KEPT DOING.
Now speak without breathing out too much.
The younger you are the longer it goes for, as my son Dempsey sounded like a chipmunk for ages.
See Photography is fun!!