Special Relativity Down to Earth

Though this mirror contraption seems contrived, it's only the simplest example. In fact, all clocks slow down, mechanical, biological, and atomic. Why? Einstein also insisted that not only was the speed of light constant, but all laws of physics. Therefore the gears in the clock, the rhythm of your heart and the transition of atmoic nuclei obey the same principles as the light clock. This shouldn't be surprising because the basic physical forces which govern these common clocks also propagate at the speed of light, so exactly the same arugments would be relevant.

That is why moving clocks slow down.

Just one more example to bring the point home. Imagine you're at the center of a town with a large lighted clock tower. Facing the clock tower, you start running backward. As you run faster and faster, the second hand has to appear to move more slowly, because the light from the clock is having to catch up to you. By the time you're moving at very near the speed of light, the second hand nearly freezes, because the light from the clock tower is just barely catching up to you. In that case, it's obvious that the moving clock (relative to you) runs more slowly.