A state of mind
A state of mind is when you're so obsessed with your own thoughts you forget to breathe and also forget that people are actually talking to you.
I was in a state of mind during lunch because I was thinking about my crush and didn’t hear my friend call my name.
My sister was in a state of mind because she was planning her entire future while eating a sandwich.
My neighbor was in a state of mind because he was arguing with a pigeon about whose lawn was better.