At the end, as the Road Runner is eating the bird seed while perched on the handle of the detonator box plunger, and the handle slowly moves down until it bottoms out, whereupon the dynamite it is connected to explodes. However, the detonator box is not a switch - the plunger that is pushed down is used to spin the coils of a generator, which generates the electrical pulse that is sent down the wires to the dynamite. In order for the generator to generate the pulse, the plunger must be pushed down quickly and forcefully. The Road Runner is not heavy enough to generate the necessary pressure on the plunger to spin the generator fast enough to generate the detonation pulse.
The coyote is painting the sides of the telephone "receiver" black and suddenly the top is painted the same color without the coyote even painting it.