YOU LOOK UP and there he is. You freeze. Your heart stops. Everything stops. Who is he? The room whirls, your stomach churns, and you know just what's happened. You've taken that dizzying, delightful plunge into what can only be described as . . . love at first sight. You say, "It was love at first sight." Translation: "He matched my fantasies perfectly!" The way a boy looks, his sense of humor, even his smile may be enough to capture your heart as well as your eye. Some girls, in fact, don't get around to actually meeting the boy until after they've fallen head over heels "in love" with him. Even without talking to him, they somehow "just know." According to Michael Kimmel, a sociology professor at Rutgers University in New Jersey, "If you ask 15-year-olds, "True or false: Love at first sight is usually the truest love," more boys than girls will answer `true.' Girls seem to know better, and boys continue to believe in love at first sight until they are well into their thirties.