def word_splitter(df):
    """
    Splits given tweets and adds them another column in given dataframe
    
    Parameters
    ----------
    df: dataframe containing tweets and dates of those tweets
    
    Returns
    --------
    df: 
        modified dataframe with 'Split Tweets' added column
    """

    #split tweets in df and append them to a new list 
    split_tweets = []
    for j in df['Tweets']:
        tweet = (j.split(' '))
        split_tweets.append([i.lower() for i in tweet])
    
    #Add to new column in df
    df['Split Tweets'] = split_tweets
    
    return df