Chan realized he could solve the problem by imagining his cylinder full of ball bearings as a stack of disks, with a single layer of ball bearings on each disk.
We made a special ball, with a bell in it, so the kids can keep track of where the ball is and what it's doing by listening for it.
我们做了一个特殊的足球，里面带一个铃铛。这样通过铃铛声音，孩子们可以跟着球跑并能判断出赛场情况。
3
Pitchers were forbidden to give the ball an artificial spin or trajectory by, for instance, nicking it or spitting on it and from discolouring the ball with dirt or tobacco juice or hair grease.