My old fashioned alarm clock did not go off this morning, subsequently I was late for work. I didn't have time to change the batteries so I waited until after work. Once home, I picked up the clock and used a Phillips head screwdriver to open the back of the clock. Using my finger nail I pulled on each of the two batteries from one side until they came loose. I put the dead batteries in the used battery drawer for recycling at a later date. I grabbed two fresh batteries from the adjacent drawer. I popped the batteries, making sure the positive and negatives were properly aligned and I screwed the plate back on with the screwdriver. I had to reset the time which wasn't hard, I just turned the dial on the back until it was correct. I then reset the alarm as well so that I would not be late from oversleeping the next day as well. 
Question: Why did the batteries need changing?
They were no longer working.