Bitternuts are items used in the Trouble Brewing minigame. Players can obtain the nuts from the bitternut trees found within the minigame on Mos Le'Harmless using monkeys, which are caught in the same area. In order to retrieve a bitternut, the player must first dye the monkey in the colour of their team using red water or blue water, obtained by using either red or blue flowers with a bowl of water. They then can use the monkey with the tree. Additionally, if you have a monkeyspeak amulet, you can alter the strategy used. You can choose to either make the monkey angry by taunting it, or caution him. By making him angry, he'll be more inclined to get up the tree as fast as possible and start pelting you with bitternuts, speeding up the time before you get a nut. If you tell him to be careful, he'll take a little bit longer, but will be more aware of other monkeys and will be more likely to win in a confrontation. This is also the most complicated task in Trouble Brewing.
According to Varrock Museum, monkeys on both Karamja and Mos Le'Harmless are very fond of bitternuts, although the nuts can only be found by players on Mos Le'Harmless.
For an exhaustive list of all known sources for this item, see here
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