Answer
Whether something is obvious depends on our common sense about it. In everyday experience, we only observe objects in motion with speeds much smaller than speed of light. We have no experience with and hence no common sense about, objects moving close to the speed of light. Thus it is not obvious that moving clocks don't run slow or that length don't shorten when the speeds involved approach the speed of light.
Work Step by Step
Whether something is obvious depends on our common sense about it. In everyday experience, we only observe objects in motion with speeds much smaller than speed of light. We have no experience with and hence no common sense about, objects moving close to the speed of light. Thus it is not obvious that moving clocks don't run slow or that length don't shorten when the speeds involved approach the speed of light.
