import greenfoot.*;
//Any other import statements.

public class YourWorldName extends World
{
    private boolean readPause = true;
    public boolean isPaused = false;
    //Any other fields.

    public YourWorldName()
    {    
        //Initialization code.
    }
    
    public void act()
    {
        if(!readPause && !Greenfoot.isKeyDown("p"))
            readPause = true;
        if(Greenfoot.isKeyDown("p") && readPause)
        {
            if(isPaused) isPaused = false;
            else         isPaused = true;
            readPause = false;
        }
        if(!isPaused)
        {
            //Original contents of your act() method (if any).
        }
    }
    
    //Any other methods.
}

public void act() 
{
    YourWorldName theWorld = (YourWorldName) getWorld();
    if(!theWorld.isPaused)
    {
        //Original contents of your act() method (if any).
    }
}