To start
                                                                                                                                            
                                                                                                                                                  you need to invent             gravity;
                                                                                                                                            
                                                                                                                                            and give it       time to work its way with things.
                                                                                                                                            
                                                                                                                                            Then you find a      slope and add some       water,
                                                                                                                                            
                                                                                                                                                        lots       of it
                                                                                                                                            
                                                                                                                                                  like from a       dying       glacier.
                                                                                                                                            
                                                                                                                                                              Congratulations!
                                                                                                                                            
                                                                                                                                                 You’ve just made a             river.