This trick is good to try in World 2-3 after beating Tryclyde, where you'll have a door and lots of Mushroom Blocks to work with. First, put a Mushroom Block on top of the door. Then, pick up another Mushroom Block, and as you're moving, throw your Mushroom Block when it's lined up with the Mushroom Block sitting on the door. If you hit the right spot, it should disappear into the Mushroom Block sitting on the door.