Two girls stared directly at me without looking away?

I was sitting at lunch with my friend who got up to get some food. I looked forwards to see these two girls sitting opposite were staring directly at me.

I tried not paying attention but it was so uncomfortable as one of them had turned their back to look at me. There was nothing behind me either so they were definitely looking at me.

When I looked at them they just kept staring for like another 10 seconds before they turned back to each other. Even afterwards they kept glancing back.

I honestly have no idea what they were doing.
Two girls stared directly at me without looking away?
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