Archaeologists recently found a 2,700-year-old pot stash, so we know humans have been smoking weed for thousands of years. But it was only about 20 years ago that neuroscientists began to understand how it affects our brains. Scientists have known for a while that the active ingredient in cannabis was a chemical called delta-9-tetrahydrocannabinol, or THC for short. Ingesting or smoking THC has a wide range of effects, from the psychoactive "getting high" to the physiological relief of pain and swelling. It also acts as both a stimulant and depressant. How could one substance do all that? Meet the cannabinoid receptor In the 1980s and 90s, researchers identified cannabinoid receptors, long, ropy proteins that weave themselves into the surfaces of our cells and process THC. They also process other chemicals, many of them naturally occurring in our bodies. Once we'd discovered these receptors, we knew exactly where THC was being processed in our bodies and brains, as well a...