diff --git a/technical_tests/sand_grain_drop.py b/technical_tests/sand_grain_drop.py
new file mode 100644
index 0000000..12d2ec6
--- /dev/null
+++ b/technical_tests/sand_grain_drop.py
@@ -0,0 +1,92 @@
+#!/usr/bin/env python3
+# coding: utf8
+
+"""
+Author: freezed <git@freezed.me> 2020-07-16
+Licence: `GNU GPL v3` GNU GPL v3: http://www.gnu.org/licenses/
+
+This file is part of [`free_zed/mypsb`](https://gitlab.com/free_zed/mypsb/)
+"""
+
+
+def main(pile, n):
+    """
+    This function returns the situation of a sand `pile` after droping `n`
+    sand grain on top of it.
+
+
+    1. Sand pile is a square table of uneven size (viewed from up)
+    1. Sand grain is always dropped on center of pile.
+    1. When a cell had 4 grains inside, these grains moves on the near 4th cells
+    1. Grains going out the pile are losts
+
+    :Tests:
+    >>> main([[1,1,1],[1,1,1],[1,1,1]],1)
+    [[1, 1, 1], [1, 2, 1], [1, 1, 1]]
+    >>> main([[1,1,1],[1,1,1],[1,1,1]],2)
+    [[1, 1, 1], [1, 3, 1], [1, 1, 1]]
+    >>> main([[1,1,1],[1,1,1],[1,1,1]],3)
+    [[1, 2, 1], [2, 0, 2], [1, 2, 1]]
+    >>> main([[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1]],1)
+    [[1, 1, 1, 1, 1], [1, 1, 1, 1, 1], [1, 1, 2, 1, 1], [1, 1, 1, 1, 1], [1, 1, 1, 1, 1]]
+    >>> main([[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1],[1,1,1,1,1]],3)
+    [[1, 1, 1, 1, 1], [1, 1, 2, 1, 1], [1, 2, 0, 2, 1], [1, 1, 2, 1, 1], [1, 1, 1, 1, 1]]
+    >>> main([[0,0,3,0,0],[0,0,3,0,0],[3,3,3,3,3],[0,0,3,0,0],[0,0,3,0,0]],1)
+    [[0, 1, 0, 1, 0], [1, 2, 2, 2, 1], [0, 2, 0, 2, 0], [1, 2, 2, 2, 1], [0, 1, 0, 1, 0]]
+    """
+
+    size = len(pile)
+    center = int((size - 1) / 2)
+
+    while n != 0:
+        pile[center][center] += 1
+        do_propagation(pile)
+        n -= 1
+
+    # All sand grain are dropped, let's finish propagations
+    while True in [4 in row for row in pile]:
+        do_propagation(pile)
+
+    return pile
+
+
+def do_propagation(pile):
+    """
+    Walk through the pile to propagate sand grains
+
+    :tests:
+    >>> test_case = [[0,0,4,0,0],[0,2,0,2,0],[4,0,4,0,4],[0,2,0,2,0],[0,0,4,0,0]]
+    >>> do_propagation(test_case)
+    >>> print(test_case)
+    [[0, 1, 0, 1, 0], [1, 2, 2, 2, 1], [0, 2, 0, 2, 0], [1, 2, 2, 2, 1], [0, 1, 0, 1, 0]]
+    """
+
+    size = len(pile)
+    x_max, y_max = size - 1, size - 1
+
+    # find cells > 3
+    for x in range(size):
+
+        for y in range(size):
+
+            if pile[x][y] > 3:
+                pile[x][y] -= 4
+
+                # update west neighbor
+                if x < x_max:
+                    pile[x + 1][y] += 1
+                # update east neighbor
+                if x > 0:
+                    pile[x - 1][y] += 1
+                # update north neighbor
+                if y < y_max:
+                    pile[x][y + 1] += 1
+                # update south neighbor
+                if y > 0:
+                    pile[x][y - 1] += 1
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()