Go to Wet-Dry world (or that place where the water level differs). Jump into the level with a high jump (either a sideways flip or one of the woo-woohoo-waha things) so that the water level starts out high. Now, swim over to the cannon above the water. It's near a corner of the level, and is on a jutting-out platform. Jump in. Now, aim the cannon at the sun and place it as high as you can from the center of it. Shoot out. If you did it correctly, you will bash into the invisible ceiling, hit the ground and lose half of your life, and slide into the cannon. Repeat this one more time and you will have no life, but be in the cannon. Shoot yourself anywhere... once you land, you will immediately die. If you shoot yourself into the water, you will drown. If you shoot yourself into the one-up in the tunnel you'll die but gain a life, but then lose a life from your death.