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+# Mutable vs immutable objects
+
+Imagine that you want to make all letters in a string upper case.
+Conveniently, strings have an `.upper()` method.
+
+You might think that this would work:
+```python
+string = "abcd"
+string.upper()
+print(string) # abcd
+```
+
+`string` didn't change. Why is that so?
+
+That's because strings in Python are _immutable_. You can't change them, you can only pass
+around existing strings or create new ones.
+
+```python
+string = "abcd"
+string = string.upper()
+```
+`string.upper()` creates a new string which is like the old one, but with all
+the letters turned to upper case.
+
+`int`, `float`, `complex`, `tuple`, `frozenset`  are other examples of immutable data types in Python.
+
+Mutable data types like `list`, on the other hand, can be changed in-place:
+```python
+my_list = [1, 2, 3]
+my_list.append(4)
+print(my_list) # [1, 2, 3, 4]
+```
+
+`dict` and `set` are other examples of mutable data types in Python.