Swiss chemist Albert Hofmann first synthesized the psychedelic drug Lysergic acid dethylamide (LSD), commonly known as acid, on Nov. 16, 1938, but it was not until April 19, 1943, that he intentionally took the drug and experienced his first trip as he was riding his bicycle home through the streets of Basel.

That day of discovery has come to be known as Bicycle Day, a term that has its origins in the counter-culture of the 1960s, when use of LSD became increasingly widespread.

The story goes that Hofmann was working for the pharmaceutical company Sandoz, experimenting with developing various chemical drugs derived from the fungus ergot, which was known to cause hallucinations, but had many other practical medical applications, such as quickening labor and preventing prenatal bleeding.

Hofmann synthesized LSD without ever learning about its psychoactive properties during some of the most tumultuous years of World War II. He shelved it away and decided to reexamine the drug five years later on April 16, 1943, although peace was still some years off.

During that experiment, he was exposed to a small dosage of the drug and experienced some of its hallucinogenic effects. Technically, that was his first acid trip, but it was three days later that he intentionally ingested a larger dose, 0.25 milligrams, and went on the fabled psychedelic bicycle ride.

Everything in my field of vision wavered and was distorted as if seen in a curved mirror, Hofmann recalls of the event in his book, LSD - My Problem Child. I also had the sensation of being unable to move from the spot. Nevertheless, my assistant later told me that we had traveled very rapidly.

Finally, we arrived at home safe and sound, and I was just barely capable of asking my companion to summon our family doctor and request milk from the neighbors, he continues. In spite of my delirious, bewildered condition, I had brief periods of clear and effective thinking -- and chose milk as a nonspecific antidote for poisoning.

The acid trip is highly dependent on the mental state of the user and the environment in which it is experienced, which can make it positive or negative. There is no telling what someone will experience, but they are typically hallucinogenic amplifications of inner thoughts and external stimuli, particularly lights and sounds.

Scientifically, it can be as such:

What LSD does is it hijacks some of the receptors in the brain for one of the chemicals called serotonin, said Professor Philip Strange at Reading University in an interview with the Naked Scientists. Serotonin is a neurotransmitter - a brain chemical which is very important for transmitting messages in our brain. It's involved in all sorts of different things like emotion and constricting arteries as you've said. It acts by binding to proteins called receptors. LSD basically binds to those receptors and hijacks and affects it. It's not surprising it has these complex effects.

Hoffman has described an LSD trip as experiencing reality without an ego. Such an experience has been akin to epiphanic revelations and explosions of creativity. Just listen to the Beatles' music in the late sixties. On the other hand, it can also intensify the fears and destructive thoughts of those who would already be considered mentally unstable otherwise. Just look at Charles Manson.

And remember, if you're going to ride a bike, be sure to wear a helmet.