xkcd #3120: Geologic Periods

Title text:

Geologists claim it's because the earlier Cenozoic used to be called the Tertiary, but that's just a ruse to hide the secret third geologic period, between the Neogene and the Quaternary, that they won't tell us about.

Transcript:

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Source: https://xkcd.com/3120/

explainxkcd for #3120