input: Write a python function to find the most frequent words in a sentence.
  assert most_frequent_words("The quick brown fox jumps over the lazy dog") == [('the', 2)]
  assert most_frequent_words("apple apple orange banana banana banana") == [('banana', 3)]
  assert most_frequent_words("this is a test this is only a test") == [('this', 2)]
output: def most_frequent_words(sentence):
      from collections import Counter
      words = sentence.lower().split()
      count = Counter(words)
      max_freq = max(count.values())
      return [(word, freq) for word, freq in count.items() if freq == max_freq]