Minecraft wiki entry for Block_and_item_duplication, associated text: Whenever the dragon lands on the end portal and releases the purple fog, be above or below the purple fog, look at the fog at a steep angle, and proceed to harvest the dragon's breath. The fog will first shrink as intended, but after it reaches the smallest size, it will expand with each bottle used, and keep expanding until the fog disappears as too much time passed. (YouTube by Underscored Frisk)