Quote from Phoebe in The One with Ross's Denial
Ross: It was a hug. Phoebe: All right, just tell me this. Did you, or did you not, smell her hair? Ross: Smell her hair? What if I did? Phoebe: Ninety percent of a woman's pheromones come out of the top of her head. That's why women are shorter. So that men will fall in love when they hug them. Come on, Ross, you're a scientist.