When the project you are working on fails, and you have to start again, you go back to the drawing board. Example: "It looks like my plan to kill the weeds in the garden has failed. Back to the drawing board."

The drawing board is the place where a designer begins with an idea and builds it into a plan. If your plan is not successful, then you have to go back to the drawing board to start again and think up a new plan. Example: "How is your science project coming along?" Reply: "It didn't work at all!" Answer: "Well, back to the drawing board."