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A Fermented Spider Eye is crafted from a Spider Eye, a Brown Mushroom and Sugar.

A Brown Mushroom is a small fungus that grows in dark places and gives off a faint glow.
It can be composted: each one has a 65% chance to add a layer to a Composter. It gives off light level 1.
It spreads slowly in low light, and grows on mycelium and podzol in any light. Bone meal turns it into a huge brown mushroom. It goes into mushroom stew, rabbit stew, suspicious stew and fermented spider eyes.
This item has no crafting recipe.
A Fermented Spider Eye is crafted from a Spider Eye, a Brown Mushroom and Sugar.
Mining the block drops it. Sold by Wandering Trader. Shearing: Bogged and Mooshroom.
It is a food ingredient and grows huge mushrooms. It is an ingredient in 4 recipes, made at the Crafting Table; for example Suspicious Stew.
Brown Mushroom Block (22%).
Yes. Each Brown Mushroom put in a Composter has a 65% chance to add a layer (the first layer in an empty Composter always works). After 7 layers the Composter gives Bone Meal.
Yes, light level 1.