input: Write a function to explain the difference between a `list` and a `tuple` in Python. If the input is "list", return the string "A list is mutable, meaning its contents can be changed, and it is defined with square brackets." If the input is "tuple", return the string "A tuple is immutable, meaning its contents cannot be changed, and it is defined with parentheses." Otherwise, return "Invalid input."
  assert explain_difference("list")=="A list is mutable, meaning its contents can be changed, and it is defined with square brackets."
  assert explain_difference("tuple")=="A tuple is immutable, meaning its contents cannot be changed, and it is defined with parentheses."
  assert explain_difference("dictionary")=="Invalid input."
output: def explain_difference(input_type):
      if input_type == "list":
          return "A list is mutable, meaning its contents can be changed, and it is defined with square brackets."
      elif input_type == "tuple":
          return "A tuple is immutable, meaning its contents cannot be changed, and it is defined with parentheses."
      else:
          return "Invalid input."