A cube, hidden in the hand of a member of the public, it suddenly turn into two.
Take three cubes of sponge your magic kit and hides one under the fold of your thumb. The other two are placed on the table. Then ask an audience member to choose one for you and take it with the same hand where you take the other hidden. Put the new cube above the other and hide them in the hand of one of the members of the public (asking him to keep the fist tightly closed and do not open it until you do not tell). Now take the bucket left on the table and with the same hand wand. While you do this, (the cube dropped without the public to see, but keep tightly closed around the wand fist. Now place your sealed envelope hand member of the public and with a flourish of his wand fist ask Him open hand. the two cubes of sponge should appear in your hand and when you open your hand, they will see that it is empty.