def split_up_digits(number):
    """
    split up a single number expressed as a string into individual digits.
    ex. "12" becomes "1 2".
    :param number: the numerical string to split.
    :return: the new string.
    """
    new_str = ""
    for char in number:
        if len(new_str) != 0:
            new_str += f" {char}"
        else:
            new_str = char
    return new_str