That's an interesting Pavlovian interpretation! I never thought of it that way, but I guess clouds are the conditioned stimulus. It's not a totally arbitrary pairing, though. Sometimes stimuli can be paired arbitrarily, like the classic case of Pavlov's dogs pairing a bell sound with the anticipation of food. But in this case the pairing is non-arbitrary because exhaling a cloud
does often indicate that you just inhaled something psychoactive. Of course, it is also possible to do that and not exhale a cloud - but if you do exhale a cloud, you almost certainly inhaled something other than normal air, which means it could be psychoactive. Then there are all the times you breathe normal air and don't exhale clouds. From a probabilistic perspective, the brain assumes if you're breathing clouds, you're most likely altering your consciousness. Haha