求助！为什么pandas.DataFrame.dropna()在这无效了？？

import math
import numpy as np
import pandas as pd

df = pd.DataFrame({"name": ['Alfred', 'Batman', 'Catwoman'],
                   "toy": [np.nan, 'Batmobile', 'Bullwhip'],
                    "born": [pd.NaT, pd.Timestamp("1940-04-25"),
                            pd.NaT]})
print(df.head())
df.dropna()
print(df.head())import math
import numpy as np
import pandas as