So, what you need to know is not when the space key is up or down, but when it goes up or down. I know two ways of handling that. One is by using the Greenfoot.getKey() method, which only returns a key when it is "entered" (during the release of the key), as opposed to using the Greenfoot.isKeyDown(String) method which only tells you the current state of the key. It does not tell you when the key changes its state from up to down or down to up. The other way is to actually track the state of the key by using a boolean variable which can be called spaceDown. Put one in each class that uses the space key: We will have the behavior that the space key will trigger the action (changing worlds) when the key goes from an up state to a down state. So, setting it to true will prevent it from triggering until the key is in a raised state and pressed again. Another way to handle that part is to add the following line into the constructor, if a world class, or either the constructor or the addedToWorld(World) method, if an actor: In all those classes, the code to trigger on key state change from up to down goes as follows: