What is the secret ingredient to slurm in Futurama?

The secret ingredient of slurm is revealed in S02E06 “Fry and the Slurm Factory”. If you don’t want it spoiled (it is a major plot point of the episode, but not really essential to the series as a whole), you should rather watch it. If you just want the answer, here it is. Slurm is provided by a colossal slug/worm hybrid’s (or “slurm’s” if you will) heinie. It’s kinda like a cow’s milk or a bee’s honey. This is a single component of Slurm.

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