It can be dangerous in the unlikely case there's like a crack and battery acid pours out while you're using it. I repeat - UNLIKELY.

And yes, hide it, it'll save you embarassment. But do use it. There isnt a certain way, you just have to find out what works for you, what turns you on, what speeds u like, etc. If you're a virgin [which i presume you are since you're 15] I dont think it'd be the best idea to use a vibrator...unless you're comfortable with ripping your own hymen.

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Experience is the name everyone gives to their mistakes.