def give_figure(n, singular, plural=None):
    """Combine a number with a unit, whose word can be singular or plural.

    Parameters
    ----------
    n : int
        The number.
    singular : str
        The singular word.
    plural : str
        The plural word. Default: the singular with a final 's'.

    Returns
    -------
    str
        The number and the word.

    Examples
    --------
        >>> give_figure(1, 'apple')
        '1 apple'
        >>> give_figure(2, 'apple')
        '2 apples'
        >>> give_figure(1, 'equilibrium', 'equilibria')
        '1 equilibrium'
        >>> give_figure(2, 'equilibrium', 'equilibria')
        '2 equilibria'
    """
    if n <= 1:
        return str(n) + ' ' + singular
    elif plural is None:
        return str(n) + ' ' + singular + 's'
    else:
        return str(n) + ' ' + plural
