LSD is a molecule that is shaped a lot like serotonin. Serotonin is a very important neurotransmitter involved with mood and a whole lot of other things. There are receptors in your brain that are configured to receive serotonin. And as it happens, the LSD molecule fits into those receptors.
What if LSD connects your brain to other dimensions. Yeah, what if? Maybe it does. Maybe it did with your brain.
