One Christmas my mother, who was earning almost nothing, spent way more money than she could afford, went into debt, to buy me a bunch of expensive toys.
My grandfather, her father, taught her a lesson she never forgot. He made me one of these, and did not spend a penny. They tell me I played with that thing all day long, and Mom cried the whole time.
It's just an oatmeal box with some stones sealed inside, attached to a string.