<?xml version="1.0" encoding="UTF-8"?> <!--#defining a XML file-->

<launch> <!--open a tag called launch because this is also a launch file for ROS-->
    <!-- overwriting these args -->  <!--define some arguments-->
  <arg name="debug" default="false" /> 
  <arg name="gui" default="true"/>
  <arg name="pause" default="true"/> <!--the simulation is going to start it paused-->
  <arg name="world" default="$(find my_simulations)/world/empty_world.world" /> <!--define the world name look into our my_simulations package--> 

  <!-- include gazebo_ros launcher -->
  <include file="$(find gazebo_ros)/launch/empty_world.launch"> <!--include this launch file from gazebo_ros package--> 
      <arg name="world_name" value="$(arg world)"/> <!--use the arguments that we have defined before-->
    <arg name="debug" value="$(arg debug)" />
    <arg name="gui" value="$(arg gui)" />
    <arg name="paused" value="$(arg pause)"/>
    <arg name="use_sim_time" value="true"/>  <!--all the other ROS nodes are going to use the simulation time as reference.--> 
  </include>

</launch> 

<?xml version="1.0" ?>  <!--we are defining a xml file-->
<sdf version="1.5">
  <world name="default">    
    <!-- A global light source -->  
    <include>            <!--include some models like sun plane -->
      <uri>model://sun</uri>
    </include>
    <!-- A ground plane -->
    <include>
      <uri>model://ground_plane</uri>
    </include>
  </world>
</sdf>