diff --git a/spheroid.cpp b/spheroid.cpp
index ef9f381..2c0a60b 100644
--- a/spheroid.cpp
+++ b/spheroid.cpp
@@ -51,3 +51,10 @@ void Spheroid::check_vertical_plan_collision(double& abscissa) {
         }
     }
 }
+
+
+double Spheroid::operator() (double _x, double _y, double _z) const {
+            return (pow(_x - center.x, 2) / pow(q, 2)
+                  + pow(_y - center.y, 2) / pow(q, 2)
+                  + pow(_z - center.z, 2) / pow(p, 2) -1);
+        }
diff --git a/spheroid.hpp b/spheroid.hpp
index 66b2aec..ca74894 100644
--- a/spheroid.hpp
+++ b/spheroid.hpp
@@ -19,11 +19,7 @@ class Spheroid : public ImplicitSurface {
         void set_stiffness(size_t _stiffness);
         void check_horizontal_plan_collision(double& height);
         void check_vertical_plan_collision(double& abscissa);
-        double operator() (double _x, double _y, double _z) const {
-            return (pow(_x - center.x, 2) / pow(q, 2)
-                  + pow(_y - center.y, 2) / pow(q, 2)
-                  + pow(_z - center.z, 2) / pow(p, 2) -1);
-        }
+        double operator() (double _x, double _y, double _z) const;
     private:
         /**
          * p corresponds to the half-height of the ball,