Go to Good Egg Galaxy (the first level of the game, accessible at the Terrace). Near the beginning, there is a pipe with a warp star over it. Jump up to the star, but don't spin Mario, and let him drop down. He will hit the bottom of the pipe, but not enter it. It will look as if Mario is grabbing onto the edge, but you can't drop into the pipe. If you try moving around, he will sound as if he is trying to lift himself out. To get out, just jump out.
Submitted by: kornyhatemachine