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From Terraria Wiki
Placed Cobwebs look like a gray mesh that clings to walls and ceilings.
Cobwebs are crafting materials used to craft Silk and other items. Cobwebs can also be placed. They will catch players who fall into them and reset fall damage. They break after a short time when a player is stuck in them, but can also be harvested instantly with any weapon or swung tool, just like Plants.
Cobwebs are commonly found in underground areas, in clusters of 5 or more, alongside the edges of walls or ceilings. They are also found in large quantities in Spider Nests, where they regrow over time, offering players an unlimited supply.
Cobwebs have no sell value, but can be crafted into silk, which has a fairly high value of 2 .
Because of that they can be used by early game players to earn a lot of money.
If you clear an entire Spider Nest you should get about 200 cobwebs or 50 .
- Placed cobwebs can support falling blocks like sand and silt; removing the cobwebs will allow them to fall, causing damage to whatever they hit.
- Placed cobwebs don't impede the movement of monsters, NPCs or critters.
- Because they slow player movement, Cobwebs can be used in the construction of pits to prevent fall damage.