I was sitting at my desk fiddling with a pile of matchsticks when my little granddaughter came in. She stared at me across the desk and asked what I was doing. I explained that I was using the sticks to form numbers, like the numbers on the digital clock nearby. She said, "That's easy." She looked at the clock and formed a "2." I told her that I really wanted to make some equations, like "2 + 2 = 4." "Oh, that's too easy," she said. "Let me make a harder one."

I was called away for a moment, but when I sat down at the desk again, I saw this equation:

I asked her if she was sure that the equation was correct. She assured me it was and proceeded to show me. How did she prove that the equation was right?