Jointed-up thinking?Phanie / Alamy Stock PhotoBy Joshua SokolAn erratic driver is pulled over by the police. The officer smells a hint of marijuana, so dabs a cotton swab in the driver’s mouth to collect some saliva. Just 3 minutes later, still by the side of the road, the result comes back: every millilitre of spit contains 5 nanograms of THC, weed’s...