Ok, so it's not the most vital survival skill (making ink) but it's easy to learn and something most people don't know how to do. Shaggy Mane Inky Caps are edible (at the "all white" stage) and useful. The ink can be used for education, writing messages, deciphering code, documenting history, transferring ownership (a living will or deed to a property), and keeping inventory. The ink costs nothing to make and only two items are needed to utilize it properly, a container to hold the ink and a writing instrument such as a quill or paintbrush. Since it has so many uses and little to no cost, it's a great bartering resource.