input: Write a function to find the most frequent words in a given string.
  assert most_frequent_words("apple banana apple orange banana apple") == [['apple', 3], ['banana', 2], ['orange', 1]]
  assert most_frequent_words("cat dog cat bird cat dog") == [['cat', 3], ['dog', 2], ['bird', 1]]
  assert most_frequent_words("hello world hello hello world hi") == [['hello', 3], ['world', 2], ['hi', 1]]
output: def most_frequent_words(s):
      from collections import Counter
      
      words = s.split()
      counter = Counter(words)
      most_common = counter.most_common()
      
      return [[word, count] for word, count in most_common]