The third annual Philadelphia Science Festival kicks off in earnest this weekend.

More than 100 events, many of them free, will be offered throughout the city until April 28.

Science Festival director Gerri Trooskin said organizers have ironed out some of the kinks after two years of running the festival.

"Having a couple of scientists sit on a panel and just talk at an audience doesn't leave people inspired all the time," Trooskin said. "So every single one of our events has a hands-on component to some degree."

Also new this year: Visitors can sort though a sometimes daunting list of events by category ("Foodies," "Freaks and Geeks" and "Techies" among them) when searching on the Science Festival's website.

The festival includes a plethora of events on the science of food and drink, a chance to play forensic investigator at the Mutter Museum, and a lecture on the history of hallucinogens.

At that event, on Sunday at Frankford Hall, University of Pennsylvania neuroscience doctoral candidate will lecture on the history of medical research in psychedelics.

Young said, not surprisingly, the 1960s were a heyday of research into psychedelic drugs.

"There was all this research going on, a lot of exuberance surrounding what they're finding, and maybe not using the most stringent of research methods," Young said. "And then there's sort of this pullback in response to that."

When hallucinogens became inextricably tied to trippy counterculture, LSD and psilocybin, the active ingredient in hallucinogenic mushrooms, became taboo topics for serious academic research. Recently, though, the field has been experiencing something of a rebirth.

"It started as just a few research groups, now in the last couple years more and more research is starting to be done," Young said, "as people are sort of paving the way."

Researchers have won permission to study psychedelics as therapies for patients who are terminally ill or suffer from depression or obsessive-compulsive disorder, often in conjunction with psychotherapy.

Small, tightly controlled experiments have been done at the University of California Los Angeles, Johns Hopkins and New York University, among other places.