There's a certain note the human voice can register that will actually shatter glass. It's not a comfortable note to hear - opera singers train to carry that note - but children are able to hit that note at will, particularly girls (I speak from experience with my own daughter). You know you're doing a good job on Halloween night if you can get a trick-or-treater to hit that note at least once.
The first time I ever scared a trick-or-treater so bad they hit the glass-shattering note was back the mid 90's. At the time my brother and his wife lived down the street from my mom and dad, and I was helping them out on Halloween night. My costume was pretty elaborate. I had a dimestore Frankenstein Halloween mask on, my shabby clothes were rescued from the trash bin (with a bit of subterfuge in mind, I stuffed newspapers into my sleeves and pant legs so they stuck out a bit, making me look like I was stuffed.)
Earlier that day I had driven an old rusty hatchet into a plank of wood. Now it was time to put that hatchet to the test. Laying down on the boulevard between the street and the sidewalk in front of my brother's house, I placed the hatchet board on my chest and then buttoned my old ratty shirt around it. As the first trick-or-treaters came up the block I lay still. Very still. It was a cold, cold Halloween that year and it didn't take long before there was frost on the ground. I felt like I was literally starting to freeze. But I couldn't abandon my job until I had got in at least one good scare.
It was dark now, the streets were busy with kids begging for candy. I heard a group of older kids approaching and knew I had a shot at glory. As they came near I didn't even breathe. It was a group of four young girls - I would guess in the twelve to thirteen year old range. Prime targets for terror.
They walked up to me. They studied me, debating whether I was real or not. One of them even kicked my boot. "It's totally fake," the girl said. That's when I started quivering all over and grasping at her pant-legs.
You could hear that ear-piercing, glass-shattering scream for miles.