In 1-4, go past the first Firebar and stop on the next platform, where 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 the wall. Next, walk all the way to the left and jump. Mario should hit his head on the ceiling and get stuck in the wall. From there, hold Left on the Control Pad and jump repeatedly until Mario is pulled through the wall. Now you'll be able to walk through to the end of the low ceiling (beware of Firebars). It is possible to do this trick anywhere you find similar conditions.