这一小节学习THREE.js中的物理模型。什么是geometry？(英文解释，翻译为中文就看不懂了，直接看英语吧)Composedofvertices(pointcoordinatesin3Dspaces)andfaces(trianglesthatjointhoseverticestocreateasurface)Canbeusedformeshesbutalsoforparticles