Once, a couple of weeks after my first horse moved from a boarding barn to live on my property, my non-horsey dad was trying to hand feed him hay over our half/door style stall. Pause.
This gelding was all business, all the time. He was a good bit head shy, and wouldn't so much as lick your hand if it was dripping in maple syrup.
Dad held the handful of hay up to my horse's muzzle, and the horse nibbled, then bit to pull the hay out of dad's hand. He got three of Dad's fingers with the hay. Before Dad could even yell in pain, my pony had jumped clear back against the back wall of his stall, eyes bugging, and snorting. Poor fella knew he'd gotten something besides hay! Dad was so impressed he wen't right back to trying to feed him. We didn't know a lot of that pony's history, but he was petrified of a short crop near his head for a long time. Something that was very firmly ingrained in his head was basic manners. I've never before or since seen a horse look that apologetic!