Super Mario Bros. 3 Tips

Get a Warp Whistle On World 1-Level 3
Near the end of the third level in the first world, there will be a white block where two enemies start out on. Stand on it, hold down, and you will fall through and be in the background. Run to the end of the level as fast as you can and you will be with Toad with a box. Open the box and a whistle will come out.