Ermitonto has it. Psychotropic chemicals have the effects the have the effects they do not because of the medium of solubility but because of the specific chemical receptors they activate in the neurons in our brains. LSD and THC fit two entirely different neurochemical receptors. THC engages the CB-1 receptors in brain cells (and CB-2 receptors in somatic cells), while LSD mates with the 5-HT2A receptor, and to a lesser extent, the SST2A receptor. It is this, and not the solubility that determine the effect. It would be easier to pass a drug test, though, since water-soluble compounds are removed from the body faster. THC stays in the body as long as it does precisely because it is fat-soluble.