If you've ever kissed someone, in earnest, you know that it can set off a hot cascade of fireworks in your brain and central nervous system. But what chemistry drives the swoon and the racing heart?

In an interview with Science Podcast, Rutgers University anthroplogist Helen Fisher explains what she's learned in research that includes putting more than four dozen "madly in love" people through fMRI brain scans. Fisher's conclusion: Homo sapiens has three brain systems for mating and reproduction: the sex drive, the passion of being in love, and attachment.

"Kissing," she says, "evolved as an all-purpose mechanism that could stimulate any one or all of these three."

To get the chemical details, listen to her interview with Robert Frederick, host of Science Podcast.