var height = prompt("How high do you want the grid? 
(1-10 is good)","10");

var width= prompt("How wide do you want the grid? 
(1-10 is good)","10");

var a_line;

var new_window = 
window.open("grid.html","looper",
"width=400,height=400");
new_window.document.writeln("<h1>A Grid</h1>");
for (var height_loop = 0; height_loop < height; height_loop++) 
{	
	a_line = "";
	for (var width_loop = 0; width_loop < width; width_loop++)
	{
		a_line+="x";
	}
	new_window.document.writeln(a_line + "<br>");
}

After asking for height and width, opening a new window, and writing a header to it, we go into a for loop. The first for loop sets a_line = "". Try doing the example without this line and see what happens. After initializing a_line, the script goes into another for loop to build a line of x's as wide as required and prints it out to the new window. This happens height times.

OK, here's your assignment: Take a look at this loop exercise and try doing it on your own before viewing source to see how it works.