What hippies have long suspected is actually (sort of) true: Trees can scream. Surprisingly, tree "language" resembles a common painkiller. According to a recent study by the National Center for Atmospheric Research (NCAR), when walnut trees are stressed, they emit methyl salicylate, which is a chemical form of aspirin. NCAR scientist Thomas Karl says that drought or unseasonable temperatures can cause plants "to produce their own mix of aspirin-like chemicals, triggering the formation of proteins that boost their biochemical defenses and reduce injury." It's believed that neighboring trees respond to the presence of the chemical by beefing up their own immune systems. Alex Guenther, coauthor of the study, explains, "These findings show tangible proof that plant-to-plant communication occurs on the ecosystem level. It appears that plants have the ability to communicate through the atmosphere." Researchers hope these findings will provide farmers and forest managers with another tool to help them assess the health of crops or forests. If the air is thick with aspirin, the trees are in trouble.