Find one of those vases that produces an infinite supply of Bob-ombs and is covered with a Mushroom Block. Take off the Mushroom Block and throw it by the vase, leaving one block of space between the Mushroom Block and the vase. Stand behind the Mushroom Block and wait for the Bob-ombs to blow up the vase. Next, take the Mushroom Block and throw it where the vase used to be. Scroll the Mushroom Block off the screen, then scroll back. Bob-ombs will emerge from the Mushroom Block!