A genuine rarity: a sex comedy with brains. Even rarer, one with smart politics--so unobtrusive you may not notice--and wonderful acting. Writer-director Alfonso Cuaron--best known here for two Hollywood efforts, the enchanting A Little Princess and the less enchanting Great Expectations--went back to his native Mexico to put together this road movie about two 17-year-old boys from Mexico City, one privileged, the other working-class. On an impulse, they take off for a remote coastal beach with a 28-year-old married woman. It's not difficult to understand why this movie has been a smash success in Mexico, especially with teenagers; few films deal with teenage hormones, Latin machismo, and the complexities of friendship in such a refreshing way. The movie keeps surprising you and stays with you long after it's over. With Diego Luna, Gael Garcia Bernal, and Maribel Verdu. In Spanish with subtitles. 105 min. Century 12 and CineArts 6, Crown Village 18, Esquire, Landmark's Century Centre, North Riverside.