diff --git a/interactive.py b/interactive.py
index ed1baf6..42e07f9 100644
--- a/interactive.py
+++ b/interactive.py
@@ -15,12 +15,13 @@ from matplotlib.lines import Line2D
 from matplotlib.figure import Figure
 from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
 from PyQt5.QtGui import QIcon
-from PyQt5.QtCore import Qt
+from PyQt5.QtCore import Qt, pyqtSignal
 from PyQt5.QtWidgets import QSizePolicy, QMainWindow, QApplication, QAction
 from PyQt5.QtWidgets import QMenu, QWidget, QVBoxLayout, QMessageBox
 from PyQt5.QtWidgets import QSlider, QHBoxLayout, QLabel, QDialog
 from PyQt5.QtWidgets import QDialogButtonBox
 
+
 from fatiando import utils
 from fatiando.gravmag import talwani
 from fatiando.mesher import Polygon
@@ -39,6 +40,9 @@ class Moulder(FigureCanvasQTAgg):
         'n: New polygon', 'd: delete', 'click: select/move', 'a: add vertex',
         'r: reset view', 'esc: cancel'])
 
+    # Signal when selected polygon changes
+    polygon_selected = pyqtSignal(float)
+
     def __init__(self, parent, x, z, min_depth, max_depth,
                  density_range=[-2000, 2000], width=5, height=4, dpi=100):
         self.fig = Figure(figsize=(width, height), dpi=dpi)
@@ -322,6 +326,8 @@ class Moulder(FigureCanvasQTAgg):
                 # and which vertice of which polygon
                 self._ipoly, self._ivert = self._get_polygon_vertice_id(event)
                 if self._ipoly is not None:
+                    # Emit signal: selected polygon changed (sends density)
+                    self.polygon_selected.emit(self.densities[self._ipoly])
                     # self.density_slider.set_val(self.densities[self._ipoly])
                     self.polygons[self._ipoly].set_animated(True)
                     self.lines[self._ipoly].set_animated(True)
diff --git a/moulder.py b/moulder.py
index 933e8e4..f3cb51e 100644
--- a/moulder.py
+++ b/moulder.py
@@ -30,13 +30,13 @@ class MoulderApp(QMainWindow):
         self.setWindowIcon(QIcon.fromTheme('python-logo'))
         self.setGeometry(200, 200, 1024, 800)
         self.init_ui()
-        self.set_callbacks()
 
         self.moulder = Moulder(self, numpy.linspace(0, 100e3, 101),
                                numpy.zeros(101), 0, 10000,
                                density_range=DENSITY_RANGE,
                                width=5, height=4, dpi=100)
         self.setCentralWidget(self.moulder)
+        self.set_callbacks()
 
     def keyPressEvent(self, event):
         keys_dict = {Qt.Key_N: "n", Qt.Key_R: "r",
@@ -68,6 +68,7 @@ class MoulderApp(QMainWindow):
             self._spin_slider_changed_callback)
         self.error_spinbox.valueChanged.connect(
             self._spin_slider_changed_callback)
+        self.moulder.polygon_selected.connect(self._change_density_callback)
 
     def _define_actions(self):
         self.configure_action = QAction(QIcon.fromTheme('preferences-system'),
@@ -166,6 +167,10 @@ class MoulderApp(QMainWindow):
             self.error_slider.setValue(self.error_slider.float_2_int(value))
             self.moulder.error = value
 
+    def _change_density_callback(self, value):
+        self.density_spinbox.setValue(value)
+        self.density_slider.setValue(value)
+
     def _quit_callback(self):
         answer = QMessageBox.question(self, "Quit",
                                       "Are you sure you want to quit?",