diff --git a/day_and_night.py b/day_and_night.py
new file mode 100644
index 0000000..643ba14
--- /dev/null
+++ b/day_and_night.py
@@ -0,0 +1,48 @@
+import numpy as np
+import time
+from lib import fft_convolve2d
+import matplotlib.pyplot as plt
+plt.ion()
+
+def day_and_night(state, k):
+    """
+    'day & night' automata state transition
+    """
+    # computes sums around each pixel
+    b = fft_convolve2d(state,k).round()
+    c = np.zeros(b.shape)
+
+    c[np.where((b == 3) & (state == 1))] = 1
+    c[np.where((b == 6) & (state == 1))] = 1
+    c[np.where((b == 7) & (state == 1))] = 1
+    c[np.where((b == 8) & (state == 1))] = 1
+
+    c[np.where((b == 3) & (state == 0))] = 1
+    c[np.where((b == 4) & (state == 0))] = 1
+    c[np.where((b == 6) & (state == 0))] = 1
+    c[np.where((b == 7) & (state == 0))] = 1
+    c[np.where((b == 8) & (state == 0))] = 1
+
+    # return new state
+    return c
+
+if __name__ == "__main__":
+    # set up board
+    m,n = 100,100
+    A = np.zeros((m,n))
+    A = 0.63*np.random.random(m*n).reshape((m, n))
+    A = A.round()
+
+    # construct convolution kernel
+    k = np.zeros((m, n))
+    k[m/2-1 : m/2+2, n/2-1 : n/2+2] = np.array([[1,1,1],[1,0,1],[1,1,1]])
+
+    # plot each frame
+    plt.figure()
+    img_plot = plt.imshow(A, interpolation="nearest", cmap = plt.cm.gray)
+    plt.show()
+    while True:
+        A = day_and_night(A, k)
+        img_plot.set_data(A)
+        plt.draw()
+        time.sleep(0.01)
diff --git a/two_by_two.py b/two_by_two.py
index dd0666b..864774e 100644
--- a/two_by_two.py
+++ b/two_by_two.py
@@ -40,4 +40,4 @@ def two_by_two(state, k):
         A = two_by_two(A, k)
         img_plot.set_data(A)
         plt.draw()
-        time.sleep(0.1)
+        time.sleep(0.01)