What's wrong with a is b in python?

Question

In python I did this experiment:

In [1]: a = 42

In [2]: b = 42

In [3]: a is b
Out[3]: True

In [4]: a = 257

In [5]: b = 257

In [6]: a is b
Out[6]: False

Can anyone explain why it shows false for the second time? It looks really odd...