更改

* Shapes of the wood ball and the stone ball are perfect balls with the diameters of 4 meters, which are fixed in scripts, which means that only the appearances, not the hitboxes of wood/stone balls can be changed by modifying Balls.nmo; the shape of paper ball is irregular and its hitbox can even be modified in Balls.nmo.

* Though the paper ball has a elasticity 4 times of the stone ball, 2 times of the wood ball, 修改Balls.nmo时将三者等比调高，制作出[[弹力球]]时却会导致石球表观弹力高于木球，木球表观弹力高于纸球。推测这可能是三者空气阻力不同所致。

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