diff --git a/src/element.py b/src/element.py
index 2e81f74..b004318 100644
--- a/src/element.py
+++ b/src/element.py
@@ -4,7 +4,7 @@ import sys
 
 from point import Point,l_infinity,l_2
 from tools import isint_nonzero,sgn,in_interval
-from kivy.graphics import Rectangle
+from kivy.graphics import Rectangle,Ellipse,Line
 
 # parent class of all elements
 class Element():
@@ -199,6 +199,13 @@ class Element_square(Element):
 # circular elements
 # (size is the diameter)
 class Element_circle(Element):
+    # draw element
+    def draw(self,painter):
+        Ellipse(pos=(painter.pos_tocoord_x(self.pos.x-0.5*self.size),painter.pos_tocoord_y(self.pos.y-0.5*self.size)),size=(self.size*painter.base_size,self.size*painter.base_size))
+
+    # draw boundary
+    def stroke(self,painter):
+        Line(circle=(painter.pos_tocoord_x(self.pos.x),painter.pos_tocoord_y(self.pos.y),self.size*0.5*painter.base_size))
 
     # check whether an element interacts with square
     # TODO: this only works if element is a circle!
@@ -227,10 +234,10 @@ class Element_circle(Element):
         R=(element.size+self.size)/2
 
         # smallest root of t^2 v^2+2x.v t+x^2-R^2
-        t=(-v.dot(x)+sqrt(v.dot(x)*v.dot(v)-v.dot(v)*(x.dot(x)-R*R)))/v.dot(v)
+        t=(-v.dot(x)-math.sqrt(v.dot(x)*v.dot(x)-v.dot(v)*(x.dot(x)-R*R)))/v.dot(v)
 
         # return difference to pos
-        return t*v
+        return v*t
 
     # move along edge of circle
     # TODO: this only works if element is a circle!
diff --git a/src/painter.py b/src/painter.py
index f2cc6a5..f9bf433 100644
--- a/src/painter.py
+++ b/src/painter.py
@@ -7,7 +7,7 @@ from kivy.core.window import Window
 from kivy.graphics import Color,Line
 
 from point import Point
-from polyomino import Cross
+from polyomino import Cross,Disk
 
 from tools import remove_fromlist
 
@@ -215,7 +215,8 @@ class Painter(Widget):
 
             # create new particle
             if touch.button=="right":
-                new=Cross(touchx,touchy)
+                #new=Cross(touchx,touchy)
+                new=Disk(touchx,touchy)
                 # snap to lattice
                 if self.lattice!=None:
                     new.move(self.lattice.nearest_delta(new.elements[0].pos))
diff --git a/src/point.py b/src/point.py
index 91dca72..1856ed4 100644
--- a/src/point.py
+++ b/src/point.py
@@ -52,4 +52,4 @@ def l_infinity(x):
 
 # L 2 norm
 def l_2(x):
-    return sqrt(x.x*x.x+x.y*x.y)
+    return math.sqrt(x.x*x.x+x.y*x.y)
diff --git a/src/polyomino.py b/src/polyomino.py
index 7487651..6f422a1 100644
--- a/src/polyomino.py
+++ b/src/polyomino.py
@@ -2,7 +2,7 @@
 from kivy.graphics import Color,Line
 
 from point import l_infinity
-from element import Element_square
+from element import Element_square,Element_circle
 
 # parent class of all polyominos
 class Polyomino():
@@ -105,3 +105,7 @@ class Cross(Polyomino):
         *(coordx-0.5*painter.base_size,coordy-0.5*painter.base_size),
         ))
 
+# disk
+class Disk(Polyomino):
+    def __init__(self,x,y,**kwargs):
+        super(Disk,self).__init__(**kwargs,elements=[Element_circle(x,y,size=kwargs.get("size",1.0))])