[None, 'Batmobile', 'Joker']))
        >>> df
           age       born    name        toy
        0  5.0        NaT  Alfred       None
        1  6.0 1939-05-27  Batman  Batmobile
        2  NaN 1940-04-25              Joker

        >>> df.isna()
             age   born   name    toy
        0  False   True  False   True
        1  False  False  False  False
        2   True  False  False  False

        Show which entries in a Series are NA.

        >>> ser = pd.Series([5, 6, np.nan])
        >>> ser
        0    5.0
        1    6.0
        2    NaN
        dtype: float64

        >>> ser.isna()
        0    False
        1    False
        2     True
        dtype: bool
        ri