I am new to ROS, when I follow the instruction to execute roscore ; rosmake turtlesim ; rosrun turtlesim turtlesim_node , I can't not find the blue "turtlesim window". The distro which I installed is jade. I also tried to execute "roscore" in a new terminal, but the turtlesim window isn't shown on the screen. Does anyone have the same problem or have any idea about this issue? Thanks a lot.