Cobweb

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The cobweb is a new item introduced in Beta 1.5. It was previously found in the terrain.png file, but had no block ID assigned to it. It is not generated or crafted in any way. Current thoughts are that spiders will nest in it, shoot the web at you to slow you down, or drop it as loot. Cobwebs are also flammable, but they can't be ignited.

Moving into a cobweb limits your movement abilities. When you are "inside" the cobweb, you will move at a speed equal to about 15% of the normal walking speed. Additionally, jumping will vibrate the player, akin to jumping in a 2 block high space. dropping an item into it will make the item stick and gradually sink through it. Unlike water, however, falling into a cobweb will not prevent fall damage, although it will be delayed until you make contact with the ground.

When cobweb is destroyed by shears or a sword, it drops 1 string.

History
Notch originally planned for spiders to produce webs. A recent photo posted by Jeb shows that there could possibly be cobwebs on randomly generated ruins but with a new look.