def add_zero(nb):
    """Converts a number to a string and adds a '0' if less than 10.

    .. code-block:: python

        pycof.add_zero(nb)

    :Parameters:
        * **nb** (:obj:`float`): Number to be converted to a string.
    
    :Example:
        >>> pycof.add_zero(2)
        ... '02'

    :Returns:
        * :obj:`str`: Converted number qs a string.
    """
    if nb < 10:
        return '0' + str(nb)
    else:
        return str(nb)
