by showing him/her a garlic, cross, or a mirror, or exposed to a daylight, that simple.
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they do not come out into the sunlight, they have pale skin, different eye colours and they always come out at night
Remind them what happened when a princess took a chance & kissed one.
What happens to you if you are exposed to garlic? Actually, vampires don't have any reaction to garlic. It won't hurt them, but they do keep away from it, as it smells like rotting vampire bodies. I...
you can tell them because they look a little diffrent to us and never go to work / school on sunny days most look pale white