In 1-4, go past the first Firebar and stop on the next platform. You'll notice that the ceiling drops down and almost forms a wall. Slowly go to the right until there's about half a block length between the left edge of the screen and that wall. Direct Mario to the left side of the screen, and jump. Mario should hit his head on the ceiling and land on the wall. (If Mario can't fit, you've scrolled too far. If Mario doesn't get stuck, you haven't scrolled far enough.) Now, hold Left and jump rapidly until Mario starts moving backwards through the wall. From here, you'll be able to walk through to the end of that low ceiling (beware of Firebars). It is possible to repeat this trick anywhere you find similar floor/ceiling/wall combinations.