Apply some torque to it so it spins.

sfmldemo/main.cpp

@@ -137,9 +137,11 @@ int main() {
           if(sf::Mouse::isButtonPressed(sf::Mouse::Left)) {
             b2Vec2 force(-200, 1000);
             box.body->ApplyForceToCenter(force, true);
+            box.body->ApplyTorque(100.0f, true);
           } else if (sf::Mouse::isButtonPressed(sf::Mouse::Right)) {
             b2Vec2 force(200, 1000);
             box.body->ApplyForceToCenter(force, true);
+            box.body->ApplyTorque(-100.0f, true);
           }
           break;
       }