If you’ve ever looked at a tree trunk and noticed patches of gray, green, blue, or even pale yellow growing across the bark, you may have wondered if the tree was sick. Good news — in most cases, those strange crusty growths are lichens, and they’re usually completely harmless. In fact, lichens are some of the most fascinating organisms in nature because they aren’t actually a single organism at all. They’re more like a tiny living partnership. A lichen is formed when a fungu