自行车链条上的齿轮必须正确啮合才能顺利转动。

The ___cog on the bike chain must mesh properly for smooth movement.

在这家工厂里，每个工人都像机器中的齿轮，虽小却不可或缺。

In this factory, every worker is like a ___cog in the machine, small but essential.