Q: Why does chocolate and peanut butter seem like such an irresistible combination?

You mean why is anything that's labeled "Reese's" the crack cocaine of candy? Partly because about 90 percent of taste is the result of smell, and in the case of peanut butter and chocolate, the complex interplay of their odors works very well. "The flavor and composition of roasted nuts are close to those of cocoa beans, but complementary," says Gregory Ziegler, Ph.D., a Penn State University professor of food science who studies chocolate. The soft nut oils add some punch too. "If they mix with the cocoa butter, it causes everything to melt more quickly in your mouth, releasing the flavor in a burst," Ziegler says. At the same time, aromatic compounds like pyrazines are also lighting up your limbic system, a brain region that controls your memories and emotions, says Cynthia Lipka, president of the Society of Flavor Chemists. This boosts your feelings of satisfaction by tapping into deep-seated memories of moments when you indulged in the combo, such as on Halloween night or that time you tripped and your chocolate fell into someone's peanut butter. For an even bigger hit, try the combination at a warmer temperature, just before it starts to melt.