What is the reason for this difference between lists and tuples?

[Athenaeum]

I am a Python beginner, and recently I discovered a key difference between lists and tuples.

If I write something like:

list1 = list1.append(list2)

...then list1 becomes a "NoneType" object. In other words if you pass an appended list to its own name, then it is replaced with a NoneType object.

This isn't the case with tuples though...is there a reason for this?