Street art, art or not?

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Across
  1. 3. to move forwards lightly and quickly making a little jump with each step
  2. 4. if the police arrest somebody, the person is taken to a police station and kept there because the police believe they may be guilty of a crime
  3. 6. made of concrete
  4. 8. person who does a job without being paid for it
Down
  1. 1. to separate into parts; to make something separate into parts
  2. 2. to break without dividing into separate parts; to break something in this way
  3. 5. to change the form of something; to change in form
  4. 7. a strong, hard metal that is made of a mixture of iron and carbon