Detailed Instructions: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Problem:Witches' Flight (Spanish: Vuelo de Brujas; also known as Witches in Flight or Witches in the Air), is an oil on canvas painting completed in 1798 by the Spanish painter Francisco Goya.
Solution:
What is the surface of Witches' Flight made of?