They knew, when their daughter left home for a dormitory room i Heidelberg, that she was the one that had used most of the hot water, the electricity and the food in the fridge. When their son also moved they could see that they almost didn't use any energy, any food or any strange things fabricated in China.
As they got used to living without their children they had a short reawakening where they went to the movies, to fancy cocktail bars and had sex in nature. But then they returned to the quiet life, working their day jobs, reading a bit in the evening as they had become bored with television.
The money on their account grew and grew even though they gave their children as much as they would accept. So one day they both stopped working.
On that day they were sitting outside their house looking into infinity, looking into a blue sky. There was no wind. The hands of the woman turned into white dust. When she noticed he was already turned into a pile of the same material.
When the children found them they put the white dust in a flowerpot.
But nothing would grow from it.