The molecules bounce off each other like two colliding billiard balls.当分子相互弹开时就象两个碰撞的弹球.A billiard - table must be perfectly even.台球桌必须十分平.The billiard ball travelled gently across the table.台球慢慢地滚过台面....
床垫是否高低不平并且下陷？Is the mattress lumpy and saggy?那意味着整个房子正在下陷吗？Does that mean the whole house is subsiding?黑色的拎包由于装了各种形状的东西而中间下陷.The black reticule sagged under the weight of shapeless objects....