The flapper or lower tank ball is not sealing well enough and the toilet tank is slowly losing water down to the bowl. Then the float or upper ball drops enough for the tank to be refilled (topped off).
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.