def descending_coin(coin):
    """Returns the next descending coin in order.
    >>> descending_coin(25)
    10
    >>> descending_coin(10)
    5
    >>> descending_coin(5)
    1
    >>> descending_coin(2) # Other values return None
    """
    if coin == 25:
        return 10
    elif coin == 10:
        return 5
    elif coin == 5:
        return 1