input: Write a python function to tokenize a string into words.
  assert tokenize("Hello, World!") == ["Hello,", "World!"]
  assert tokenize("This is a test.") == ["This", "is", "a", "test."]
  assert tokenize("Tokenize this string") == ["Tokenize", "this", "string"]
output: def tokenize(s):
      return s.split()