My roommate eats all my food, and I really don't know what to do?

So she asks me very politely if she can have it and sometiems she won't even ask; she'll just eat it. And then I give the packet to her, she takes it and eats it. Like the entire packet!
I don't know what to do? Like should I say no the next time she asks?
My roommate eats all my food, and I really don't know what to do?
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