input: Write a python function to classify tweets as positive or negative based on the presence of the words "great" or "fun".
  assert classify_tweets(["I'm having a great time!", "I hate this so much.", "This is really fun!"]) == ['positive', 'negative', 'positive']
  assert classify_tweets(["This is terrible.", "What a fun day!", "Not great at all."]) == ['negative', 'positive', 'negative']
  assert classify_tweets(["Just okay.", "Fantastic fun!", "Pretty great!"]) == ['negative', 'positive', 'positive']
output: def classify_tweets(tweets):
      classifications = []
      for tweet in tweets:
          if 'great' in tweet.lower() or 'fun' in tweet.lower():
              classifications.append('positive')
          else:
              classifications.append('negative')
      return classifications