Einstein started Special Relativity with a discussion of simultaneity based on thought experiments. Today the internet is full of people who cannot accept his analysis and the so-called twin paradox continues to intrigue people. However, today's clocks are so good that they are almost perfect substitutes for thought experiments. I will describe modern clocks to bring out this feature and I will describe experiments where people watched identical clocks measure time at different rates. Moreover, their rate difference is in agreement with well accepted principles of physics like energy conservation. Only then do I turn to Einstein's discussion and derive how time passes for different inertial observers, in other words: I derive the Minkowski indefinite geometry of special relativity. Knowing the Minkowski geometry lets the twin paradox disappear.