Warp Whistle

The Warp Whistle is a special item in Super Mario Bros. 3 that is found hidden in two levels of the Grass Land world and the Desert Land world.

Description
Similar to the Warp Pipes from the first Super Mario Bros., the Warp Whistle allows for its user to warp to any level of their choice depending on the set of levels that they use the whistles in:

For example:

If used in World 1, the player can warp to World 2,3, or 4.

If used in World 2-6, the player can warp to World 5, 6, or 7.

If used in World 7-8, the player can warp to World 8.

World 1-3
If Mario/Luigi crouch long enough on a white block, they will fall into the background and be able to run behind the scenery, as well as the black wall at the end. At the end is a Toad House containing the first Warp Whistle.

Grass Land Fortress
The second Warp Whistle is found at the Grass Land Fortress. Once Mario/Luigi obtains the Super Leaf from a block where the first door is located, they can fly up above the ceiling where the door is. After flying as far as they can at the end, they can press on the D-Pad to be warped to a room containing a treasure box with the Warp Whistle.

World 2
The final Warp Whistle can be obtained by breaking a rock at the top right corner of the screen (after defeating the Boomerang Bro. that is holding the Hammer). After breaking the rock, they will reach a secret area with a Fire Bro.. Defeating the Fire Bro. will help Mario/Luigi claim the final Warp Whistle.