This past weekend marked the third birthday of my friend’s daughter. Inspired by recent reading, I decided to make a round dice (‘die’ if you’re a grammatical stickler) as a birthday gift. The inside of the sphere is a hollow octahedron enclosing a loose ball bearing. When the sphere is thrown the ball bearing bounces about until eventually forced to rest in one of six corners. The weight of the trapped ball bearing offsets the center of mass inhibiting rolling motion of the sphere.
The sum of the opposite sides of (six-sided) dice is always seven. Here the ball bearing is trapped above #6 revealing #1 on the top: 1 + 6 = 7. The inset picture illustrates this. Amaze your friends by guessing the number at the bottom of any (six-sided) dice by simply subtracting the top number (n) from seven: 7 – n = answer.
The round dice is carved from cherry wood. The ball bearing is one of several fished from a broken pachinko machine I purchased at an antiques store with my friend fifteen years ago. The gift was enthusiastically received, although I’m not certain he’ll share it with his daughter.