1. Open the oven door and remove the crumbs and dirt that have built up on the tray.
  2. Pour a pot of boiling water onto the tray and place it in front of the oven.
  3. Place a baking sheet on top of the water and close the oven door.
  4. Bake for about 15 minutes, or until the water is hot and bubbly.
  5. Remove from heat and let cool for a few minutes before using.