A Knife is a tool used for fletching logs for experience of Fletching skill. Knife can be bought from Bob at the armory for 25 coins and it never breaks, so you only need one if you don't lose it.

To use Knife for fletching, you select the knife by left-clicking it and then left-click logs. Select logs you want to use for fletching from the pop-up and then select what do you want to make from the logs. It should now use all the specific logs in your inventory to create the item of your choice. This can be canceled by walking or performing other actions while your character is fletching.