George's Marvellous Medicine by Roald Dahl
George's Grandma is a grizzly, grumpy, selfish old woman with pale brown teeth and a small puckered up mouth like a dog's bottom. Four times a day she takes a large spoonful of medicine, but it doesn't seem to do her any good. She's always just as poisonous after she's taken it as she was before. When George is left to look after her one morning, it's just the chance he needs . . .