Click the 'Place Point' button to randomly place a point somewhere in the gray rectangle - you can use this to see how well your experimental probability matches with your theoretical probability. Click the 'Reset' button to reset the experiment. To turn on the grid and count squares to figure out the probability, right-click anywhere in the white space and click the 'Grid' menu item. You can drag the vertices of the blue shape to make it as big or as small as you like. Overlap two of the vertices to fool it into thinking it's a triangle.