input: Write a function to explain the difference between a ``list`` and a ``tuple`` in Python. If the input is "list", the function should return "A list is mutable, meaning its contents can be changed, and it is enclosed in square brackets." If the input is "tuple", the function should return "A tuple is immutable, meaning its contents cannot be changed, and it is enclosed in parentheses." If the input is anything else, the function should return "Invalid input."
  assert explain_list_tuple("list")=="A list is mutable, meaning its contents can be changed, and it is enclosed in square brackets."
  assert explain_list_tuple("tuple")=="A tuple is immutable, meaning its contents cannot be changed, and it is enclosed in parentheses."
  assert explain_list_tuple("dictionary")=="Invalid input."
  assert explain_list_tuple(123)=="Invalid input."
  assert explain_list_tuple([])=="Invalid input."
output: def explain_list_tuple(input_type):
      if input_type == "list":
          return "A list is mutable, meaning its contents can be changed, and it is enclosed in square brackets."
      elif input_type == "tuple":
          return "A tuple is immutable, meaning its contents cannot be changed, and it is enclosed in parentheses."
      else:
          return "Invalid input."