Detailed Instructions: Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Q: Context: 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.
Question: What is the surface of Witches' Flight made of?
A:
canvas