diff --git a/notes-en/python-iterators.md b/notes-en/python-iterators.md
index b10c835..79e6821 100644
--- a/notes-en/python-iterators.md
+++ b/notes-en/python-iterators.md
@@ -80,6 +80,34 @@ Out[20]: 0
 
 ## Why we need iterators?
 
+In first, iterators save your memory:
+
+```python
+In [10]: import sys
+
+In [11]: sys.getsizeof(iter(range(10)))
+Out[11]: 48
+
+In [12]: sys.getsizeof(iter(range(1000)))
+Out[12]: 48
+
+In [13]: sys.getsizeof(list(range(1000)))
+Out[13]: 9112
+
+```
+
+Also sometimes we don't need whole elements from iterable. For example, `in` stops iteration when it's found element, `any` stops on first `True` and `all` on first `False`.
+
+```python
+In [6]: %timeit 10 * 4 in [i for i in range(10 * 10)]
+2.81 µs ± 11.2 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
+
+In [7]: %timeit 10 * 4 in (i for i in range(10 * 10))
+2.07 µs ± 13.7 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
+```
+
+## Why we need iterable?
+
 Iterable, as opposite to iterator, can implements some useful methods for good performance. For example, method `__in__`for `in` operaton:
 
 ```python
@@ -90,8 +118,40 @@ In [27]: %timeit 10 ** 8 in iter(range(10 ** 10))
 5.22 s ± 5.07 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
 ```
 
+## Why this is important?
+
+You can iterate via iterator only once:
+
+```python
+In [14]: r = range(5)
+
+In [15]: list(r)
+Out[15]: [0, 1, 2, 3, 4]
+
+In [16]: list(r)
+Out[16]: [0, 1, 2, 3, 4]
+
+In [17]: i = iter(range(5))
 
+In [18]: list(i)
+Out[18]: [0, 1, 2, 3, 4]
+
+In [19]: list(i)
+Out[19]: []
+```
+
+Iterables can waste your memory:
+
+```python
+In [21]: list(range(10 ** 10))
+MemoryError:
+```
 
 ## Further reading
 
-1. https://stackoverflow.com/questions/9884132/what-exactly-are-iterator-iterable-and-iteration
+1. Stack Overflow: [What exactly are iterator, iterable, and iteration?](https://stackoverflow.com/questions/9884132/what-exactly-are-iterator-iterable-and-iteration)
+1. Python etc: [How to make iterator and iterable](https://t.me/pythonetc/149)
+1. Documentation: [About iterators](https://docs.python.org/3/tutorial/classes.html#iterators) and [generators](https://docs.python.org/3/tutorial/classes.html#generators)
+1. Documentation: [iterator API](https://docs.python.org/dev/library/stdtypes.html#iterator-types)
+1. Documentation: [Iterators HOWTO](https://docs.python.org/dev/howto/functional.html#iterators)
+1. ITGram: [WTF Python](https://t.me/itgram_channel/32)