If a girl/boy that you like very much sit beside you, what is your respond?

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If a girl/boy that you like very much sit beside you what is your respond to her/him? Just like today, a girl that i like very much sit beside, my heart is jumping very fast and faster, don't why. Did you all face this problem before and did you feel shame? At that moment, i don't know what should i do, and my brain had already become blank and empty. Please share with me your ideas that i can use it for next time, and can make myself clam down at that moment.