diff --git a/csst_proto/__init__.py b/csst_proto/__init__.py
index f102a9cadfa89ce554b3b26d2b90bfba2e05273c..ba61cccaa91c40203ec13164c69e8442e1cf681a 100644
--- a/csst_proto/__init__.py
+++ b/csst_proto/__init__.py
@@ -1 +1,6 @@
+import os
+from .top_level_interface import *
+
 __version__ = "0.0.1"
+
+PACKAGE_PATH = os.path.dirname(__file__)
diff --git a/csst_proto/data/data.csv b/csst_proto/data/data.csv
new file mode 100644
index 0000000000000000000000000000000000000000..cd92b6f2dc785d991a8126557807371575922210
--- /dev/null
+++ b/csst_proto/data/data.csv
@@ -0,0 +1,3 @@
+a, b, c
+1,2,3
+4,5,6
\ No newline at end of file
diff --git a/csst_proto/flip_image.py b/csst_proto/flip_image.py
index e69de29bb2d1d6434b8b29ae775ad8c2e48c5391..109e7a16339aa0fc78be98ae225b306e2d72f9eb 100644
--- a/csst_proto/flip_image.py
+++ b/csst_proto/flip_image.py
@@ -0,0 +1,35 @@
+import numpy as np
+from astropy import table
+from . import PACKAGE_PATH
+
+
+# the main algorithm
+def flip_image(img: np.ndarray):
+    """ flip an input image
+
+    This function uses an awesome algorithm to flip an input image.
+
+    Parameters
+    ----------
+    img:
+        the input image
+
+    Returns
+    -------
+    the flipped image
+
+    """
+    try:
+        assert img.ndim == 2
+    except AssertionError:
+        raise AssertionError("The input image is {}D not 2D!".format(img.ndim))
+    return img[::-1, ::-1]
+
+
+# a demo on how to package data
+def print_data():
+    """ print out data """
+    fp_data = PACKAGE_PATH + "/data/data.csv"
+    data = table.Table.read(fp_data)
+    print(data)
+    return
diff --git a/csst_proto/data/your_data.txt b/csst_proto/some_other_modules.py
similarity index 100%
rename from csst_proto/data/your_data.txt
rename to csst_proto/some_other_modules.py
diff --git a/csst_proto/top_level_interface.py b/csst_proto/top_level_interface.py
new file mode 100644
index 0000000000000000000000000000000000000000..aa96701c1ea1dacc21279e484059177099d17cec
--- /dev/null
+++ b/csst_proto/top_level_interface.py
@@ -0,0 +1,4 @@
+from .flip_image import flip_image, print_data
+
+
+__all__ = ["flip_image", "print_data"]
diff --git a/demo/__init__.py b/demo/__init__.py
deleted file mode 100644
index e69de29bb2d1d6434b8b29ae775ad8c2e48c5391..0000000000000000000000000000000000000000
diff --git a/demo/demo_processor.py b/demo/demo_processor.py
deleted file mode 100644
index be22f22c40a1e38d514d3a489d8353e1f7f64802..0000000000000000000000000000000000000000
--- a/demo/demo_processor.py
+++ /dev/null
@@ -1,43 +0,0 @@
-import joblib
-
-from csst.core.processor import CsstProcessor
-from csst.msc.data_manager import CsstMscDataManager
-
-
-class CsstProcPhotonAbsorption(CsstProcessor):
-    """ This processor absorbs photons """
-
-    def __init__(self, dm: CsstMscDataManager, n_jobs: int = 1):
-        super(CsstProcPhotonAbsorption, self).__init__()
-        self.dm = dm
-        self.n_jobs = n_jobs
-
-    def run(self, debug=False):
-        """ run this processor
-
-        Parameters
-        ----------
-        debug
-            if True, use debug mode
-
-        Returns
-        -------
-
-        """
-        joblib.Parallel(n_jobs=self.n_jobs)(
-            joblib.delayed(CsstProcPhotonAbsorption.run_one_chip)(this_ccd_id)
-            for this_ccd_id in self.dm.target_ccd_ids)
-        return
-
-    def prepare(self, **kwargs):
-        """ prepare the environment """
-        return
-
-    def cleanup(self):
-        """ clean up the environment """
-        return
-
-    @staticmethod
-    def run_one_chip(ccd_id):
-        """ run for one chip """
-        return
diff --git a/examples/how_this_code_will_be_used.py b/examples/how_this_code_will_be_used.py
new file mode 100644
index 0000000000000000000000000000000000000000..a1071e6759770b4dbd9b2fd5a6773ca57ce17694
--- /dev/null
+++ b/examples/how_this_code_will_be_used.py
@@ -0,0 +1,53 @@
+import joblib
+from astropy.io import fits
+from csst.core.processor import CsstProcessor
+from csst.msc.data_manager import CsstMscDataManager
+
+from csst_proto import flip_image
+
+
+class CsstProcFlipImage(CsstProcessor):
+    """ This processor absorbs photons """
+
+    def __init__(self, dm: CsstMscDataManager, n_jobs: int = 1):
+        super(CsstProcFlipImage, self).__init__()
+        self.dm = dm
+        self.n_jobs = n_jobs
+
+    def run(self, debug: bool = False):
+        """ run this processor
+
+        Parameters
+        ----------
+        debug:
+            if True, use debug mode
+
+        Returns
+        -------
+
+        """
+        img_flipped_list = joblib.Parallel(n_jobs=self.n_jobs)(
+            joblib.delayed(CsstProcFlipImage.run_one_chip)(this_ccd_id)
+            for this_ccd_id in self.dm.target_ccd_ids)
+        return img_flipped_list
+
+    def prepare(self, **kwargs):
+        """ prepare the environment """
+        return
+
+    def cleanup(self):
+        """ clean up the environment """
+        return
+
+    @staticmethod
+    def run_one_chip(dm, ccd_id):
+        """ run for one chip
+
+        this function
+        """
+        # input file path
+        fp_input = dm.l1_ccd(ccd_id, post="img.fits")
+        img_input = fits.getdata(fp_input)
+        # flip image
+        img_flipped = flip_image(img_input)
+        return img_flipped
diff --git a/demo/demo_docstring.py b/examples/how_to_write_docstring.py
similarity index 91%
rename from demo/demo_docstring.py
rename to examples/how_to_write_docstring.py
index 34a8430cb57269ccd0652fde6200c14bb07c2061..037c3680bf66660e3300d8862a23bceacd7f6244 100644
--- a/demo/demo_docstring.py
+++ b/examples/how_to_write_docstring.py
@@ -1,7 +1,7 @@
 import numpy as np
 
 
-# this function need to be finished
+# a function to be finished
 def cos_to_be_finished(x):
     # TODO: to be finished
     return
@@ -28,5 +28,3 @@ def cos(x: float = 0.):
 
     """
     return np.cos(x)
-
-