Are you quick enough to capture the blue ball without also capturing a red ball?
Using your mouse, drag and/or resize the rectangle and isolate the blue ball above/below/left or right of the rectangle. Once you resize the rectangle to the point when either left and right side or top and bottom side of the rectangle can't get any further, then you can scream "GOTCHA". Green balls are worth extra points if captured with the blue ball. Capture a red ball and you lose.