Mexican horny-teenager comedy tells of two buddies, one affluent and one not, who, to their astonishment and delight, are able to entice the ripe Spanish wife of a bumptious philanderer to accompany them on a trek to an imaginary beach named Heaven's Mouth. En route, she offers a sexual tutorial that brings the know-it-all boys to such depths of self-revelation as to uncork a geyser of vomit. (The intrusive, third-person, know-it-all narrator heightens the feeling of patronization.) Filmmaker Alfonso Cuarón, unlike the makers of American horny-teen comedies, tries at all times to serve the characters and the story rather than simply to twist the arrow off the laugh meter. Or to put it another way, he tries at all times for realism, even when trying for raunch. And the two young actors — Gael García Bernal and Diego Luna — are revoltingly convincing. But the character of the woman — the starved-looking Maribel Verdú, whose entire intake of calories appears to go straight to her bosom — is something of a contrivance, perhaps a more complex contrivance than some, but not a more believable one, for all that. There are numerous points of cultural interest along the road to the beach (which turns out to exist after all), though the pallid color and casual camerawork fail to make the most of them.