我们出售的每支圆珠笔都带有一个备用笔芯。
因为普通圆珠笔在有重力的情况下方可工作。主要是重力作用油墨，使圆珠在球座内滚动，带出笔芯内的油墨或墨水，以达到书写的目的。
Because ordinary pens rely on gravity, while you write, gravity forces the ink down onto the ball, which rolls inside its socket and spreads the ink on your writing surface.
避免了圆珠笔总是笔油用不完就要将笔芯弃旧换新，而带来了麻烦和浪费。
The trouble and the waste caused by abandoning the core of the ball pen and changing a new one without using up the ink are avoided.
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