It doesn't take much to for a guy to start thinking about sex. (Let's be real: Most guys can get erect gazing at a particularly plump tomato.) But it's a bit more tricky for women.Scientifically speaking, certain critical switches must be triggered before your partner's brain orders the release of vasoactive intestinal peptide, the chemical that increases blood flow to the pelvic area, swelling her inner and outer labia and causing her to feel the...