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The no sign or item blocker is a special tile modifier introduced in Chip's Challenge 2. The built-in editor only allows it to be placed on top of a key or item, but it can be placed independently using external tools. The no sign is an acting wall to any movable object that has the key or item, indicated by the no tile, in their inventory. If the no sign is empty, dropping an item on it will convert it to a no sign that blocks that item; otherwise, it has no effect.
No signs do not apply to yellow teleports or any static object that is not a key or item. Such tile combinations will instead act like an empty no sign with the other object acting normally, with the exception that yellow teleports cannot be picked up when underneath no signs, empty or otherwise.
There is a glitch where ghosts interact with various tiles differently when covered by no signs, for the most part treating them the way Chip would. There is another glitch where, if CC1 boot rules are enabled, CC1 boots can be picked up from underneath no signs.