And yes you should probably scream it even if it's in a whispered voice. A sharp nail is this:
With whatever finger the nail is on they point that finger in a way to get the hanging fingernail straight up. M and H created this game and love when they pull off a bit of fingernail and leave it hanging on creating a "Sharp Nail!" They always say it a loud voice right before they scratch you with it. They developed this form of entertainment in church and now it provides several minutes of giggling back and forth in all events as they use their sharp nail to try and scratch you. So watch out if you hear the words, "Sharp Nail!"