What is the Technique?

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Across
  1. 3. They fought like cats and dogs
  2. 5. She's a real firecracker
  3. 8. Dot dot dot
  4. 10. The pilot had a fear of heights
  5. 12. Chocolate is my Achilles heel
Down
  1. 1. This homework is going to take years to do!
  2. 2. Buzz, Oink, Splat, Boom
  3. 4. The stars danced in the midnight sky
  4. 6. A sign representing something other than itself
  5. 7. A symbol repeated throughout the text
  6. 9. Vivid pictures created by words
  7. 11. Fair is foul, and foul is fair