Death
They had a cat once. It was all Al's fault. He just brought it into the house one day, wet, dirty, and looking to all the world like a bundle of hair, fur, and wet grass jumbled into one stinky and hissing mess. Al said it was cute. Ed thought it looked like a dead rat.
Mother, though reluctant, let them keep it. It lived in their room, despite Ed's protests (it stank up the place).
It died just as suddenly as they had gotten it. Run over by a military truck. Its insufferable curiosity got the better of it, yet again. Al was devastated by the lost. Ed was devastated for his brother's devastation.
They gave it a pathetic funeral. Still warm and reeking of dirt and oil, they placed it into a tin box and buried it. Ed remembered Al crying. It was after that when he realized that playtime was over and it was time to be grown up.
He discovered long ago that alchemy made people happy. It fixed things, it mended them in a flash. It didn't take long for Ed to put two and two together. He was the older brother, after all—Al's happiness was his responsibility.
It was done in the darkness of the night. Hoping to surprise Al, he dug up the cat's body. There wasn't a doubt in his mind that it would work.
He remembered screaming, he remembered the attempt that had gone wrong, and he remembered the look of terror in his mother's eyes as she saw the mutilated body of the once-cat, hissing and spitting in the dim moonlight. It was she that picked up the shovel, it was she that saved him, and it was she that took him up with her warm hands and hugged him tightly.
She told him, with a tone that Ed couldn't seem to forget (but he did), that alchemy wasn't a game. She grounded him, but she promised not to tell Al.
Ed wished he had. At least Al would remember it for the both of them.