What game had Mij invented?

Mij invented a game with a ping-pong ball, which kept him engrossed for up to half an hour at a time. Maxwell had a suitcase that had become damaged, such that the lid, when closed, remained at a slope from one end to the other. Mij discovered that if the ball was placed on the high end, it would run down the length of the suitcase. It would dash around to the other end to surprise the ball. Then, it would grab it and trot off with it to the high end once more.