def form_tweets(sentences):
    """Create a tweet, or multiple tweets if the tweets are too long, out of the sentences."""
    tweet = ""
    tweets = []
    while sentences:
        if len(tweet) + len(sentences[0]) > 139:
            tweets.append(tweet)
            tweet = "(cont'd.):"
        else:
            if len(tweet) != 0:
                tweet += "\n"
            tweet += sentences.pop(0)
    tweets.append(tweet)
    return tweets