Einstein's concept of a clock and clock paradox
  A geometric illustration of the Lorentz transformations is given. According to similarity between space and time and correspondence between a ruler and a clock, like the division number in a moving ruler, the tick number of a moving clock is independent of its relative speed and hence invariant under the Lorentz transformations. So the hand of the moving clock never runs slow but the time interval between its two consecutive ticks contracts. Thus it is Einstein's concept of slowing of the hands of moving clocks to create the clock paradox or twin paradox. Regrettably, the concept of the clock that Einstein retained is equivalent to Newton's concept of absolute time that he rejected. This is a blemish in Einstein's otherwise perfect special relativity. 
Wang Guowen