True Experience from Middle School Days

Ok so i was in middle school, about 14 or 15, and me and a bunch of guys would sit in the back seats of the bus with all the nice lookin' girls. Well there was this one girl Emily, me and her were really good friends and i was sittin' next to her one day. Well i don't remember how we got on this, but she started to try and tickle me. She was tryin' to squeeze my kneecaps in some sort of ticklish technique but it wasn't workin' at all. (I remember i had this thing goin' that i was saying i was invincible cuz i wasnt ticklish on my knees, i was a weird kid ) well anyways, so i started tickling her back in the same way, squeezing her kneecaps and she smiled and started giggling lightly. Since she couldn't tickle my knees, she went for my sides, and dug her long nails raking up and down my sides through my t-shirt, this tickled!! I started laughing and squirming and then we both made a tickle truce and stopped....now that i look back on it...WHY DID I STOP!!!!!