<human>: Like most TIFUs this actually happened about 11 years ago, when I was about 15 years old.

I was on a missions trip for my church, and a lot of other missionaries, including myself and a friend, were queued up to get on a bus to do our daily thing. My friend, we'll call her S, started looking behind me at something, and upon turning around, I saw S reaching to touch something protruding out of another girl's backpack. Once my mind catches up with my eyes, I see it's an arm, and before I could say anything, the girl knew S was reaching for the arm, and she pushed the backpack backwards and went "Bah!" scaring the crap out of S.

I was working on a few hours of sleep that night, so once S and the girl laughed about it, I looked at the prosthetic arm again, then her amputated arm (I hadn't seen many in my life at that point, so I was confused), and went, "Ooooh that's so cool! How'd you do that?!" thinking it was some sort of costume. I don't know why that thought crossed my mind...

At that point, the girl sort of gave me an awkward, puzzled look, and I think she chose to pretend she didn't hear me. It wasn't until after I got on the bus that my brain finally woke up and I realized what I had seen, and said. Thank goodness I wasn't working in her group and I was able to avoid her the rest of the missions trip. I felt so bad after that.

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: I was running on low energy, saw a prosthetic and amputated arm, thought it was a costume, and proceeded to ask the amputee how she did it (meaning make the costume).