In Super Mario Bros. 3, you find these multicolored blocks everywhere that look like they're part of the background, but play it long enough and you'll discover that the White ones have special powers: Jump on one and duck for a few seconds and you'll go into the background of the stage, behind the scenery. This is important for getting the warp whistle at the end of Stage 1-3.

But, it's important to note that other blocks in the game have the same effect.

For example: Make your way to Stage 5-7 and go down the first pipe which will take you down to ground level.

At the end of this section will be a White Block. Duck on it and (quickly) jump up and go through the pipe back to the clouds. Done quickly enough, you'll still be in the "background" and completely untouchable by enemies!

But you can totally hit them, so it's a one-way street.

Just be aware that this wears off as soon as you enter the pipe at the end of the stage, so enjoy it while it lasts!