In World 3-1 or 4-7 (easiest in 4-7), find the Spring Ball that leads to a bonus area. Spring off of the ball and hold Left on the Control Pad as you're flying upwards. Instead of going to the bonus area, you might end up high in the sky of another area. (If you are carrying any Huffin Puffins from 4-7, they'll turn into red blobs with feet.) Once you see yourself falling in the sky, repeatedly flutter jump to the right until you can't go any farther, then flutter to the left as far as you can. Keep going back and forth, and eventually you'll scroll up some land. You have now arrived back in World 1-1! You'll still have everything you collected from the level you came from, and anything else you get will be counted towards the level you came from. Although you can rack up more than 100 points from playing in two levels, the game will not award you more than 100 when you pass through the goal ring.