diff --git a/pythonetc/README.md b/pythonetc/README.md
index 7e871d8..aa9c503 100644
--- a/pythonetc/README.md
+++ b/pythonetc/README.md
@@ -78,6 +78,7 @@ More:
 1. ./new-year.md                (31 December 2020, 18:00)
 1. ./str-concat.md
 1. ./bytearray.md
+1. ./join-lists.md
 
 Out of order:
 
@@ -95,3 +96,7 @@ Unicode module
 String.Template
 String module consts
 Urllib
+types.MappingProxyType
+frozenset
+https://bugs.python.org/issue36326
+str[0][0][0]
diff --git a/pythonetc/join-lists.md b/pythonetc/join-lists.md
new file mode 100644
index 0000000..4d21dd1
--- /dev/null
+++ b/pythonetc/join-lists.md
@@ -0,0 +1,49 @@
+Suppose, we have 10 lists:
+
+```python
+lists = [list(range(10_000)) for _ in range(10)]
+```
+
+What's the fastest way to do it? To have a baseline, let's just `+` everything together:
+
+```python
+s = lists
+%timeit s[0] + s[1] + s[2] + s[3] + s[4] + s[5] + s[6] + s[7] + s[8] + s[9]
+# 1.65 ms ± 25.1 µs per loop
+```
+
+Now, let's try to use [functools.reduce](https://t.me/pythonetc/357). It should be about the same but cleaner and doesn't require to know in advance how many lists we have:
+
+```python
+from functools import reduce
+from operator import add
+%timeit reduce(add, lists)
+# 1.65 ms ± 27.2 µs per loop
+```
+
+Good, about the same speed. However, reduce is not "pythonic" anymore, this is why it was moved from built-ins into `functools`. The more beautiful way to do it is uning `sum`:
+
+```python
+%timeit sum(lists, start=[])
+# 1.64 ms ± 83.8 µs per loop
+```
+
+Short and simple. Now, can we make it faster? What if we [itertools.chain](https://t.me/pythonetc/461) everything together?
+
+```python
+from itertools import chain
+%timeit list(chain(*lists))
+# 599 µs ± 20.4 µs per loop
+```
+
+Wow, this is about 3 times faster. Can we do better? Let's try something more straightforward:
+
+```python
+%%timeit
+r = []
+for lst in lists:
+  r.extend(lst)
+# 250 µs ± 5.96 µs per loop
+```
+
+Turned out, the most straightforward and simple to understand solution is the fastest one.