Why my female friend brushed her breast against me?

We were in a line to get food and I felt someone brushing their arm against my back and ten it was my female friend then we had a mini convo then she was asking me a question while I was playing on my phone then I felt her brushing her breast on my arm and I was like uhhh then I gave her my answer then she left. Why did she do that?
Why my female friend brushed her breast against me?
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