By Tracy Pepey
Explosive, burning, gotta-have-you-now connections. Some couples have them, others not so much. What makes some of Hollywood's most prominent couples -- like
Brangelina and
TomKat -- work? We've got the good word from Chemistry.com's Dr. Helen Fisher, who helped us uncover why certain Tinseltown twosomes sizzle -- and why their hookups with past partners fizzled.
NEXT: Jessica Simpson, Tony Romo, John Mayer >>
Long-Lasting Celebrity Duos slide show >>
Do you have a strange celebrity crush? Let it out!:
what's this?