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You don't have to confront a bully if you don't feel safe. But if you feel comfortable, make sure that you are facing them. Stand with good posturemaking yourself as tall as possible. Look them directly in the eyes to communicate confidence. Once you have their attention, communicate clearly to the person who's bullying and simply tell them to stop. Make sure that your voice is assertive yet calm and collected. You can simply say, "What you're doing isn't cool. Please stop." Or you could say, "I don't like how you're treating my friend. Stop." Avoid yelling or retaliating. You don't want to hurt a bully's feelings. Most bullies are struggling themselves, so treat them with respect. If you're witnessing cyberbullying, you can send the bully a private message that says you know what's going on and they need to stop.
Use confident body language when addressing someone who's bullying. Tell the bully to stop.