Question about common lewis structures film from the course

General Discussion
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Hi, in the course in the film about imat 2020 Q44 i found sentences:

  1. Carbon dioxide (CO2) is a common molecule used to confuse students because it has a linear shape, and the vectors inside the molecule cancel each other, making it nonpolar.
  2. Methane (CH4) has a tetrahedral shape, and its vectors cancel each other as well.
    which i do not understand- what cancel each other mean here?