The twin paradox is very interesting as it demonstrates the incompleteness of the sepecial theory of relativity. Specifically, the theory is inadequate to describe accelerating coordinate frames.
The Paradox
Let's consider two twins. The first, twin A, stays on Earth while his twin brother, twin B, leaves Earth for a voyage. As measured by twin A, the voyage is composed of two segments. Over the first segment, twin B travels away from earth at a speed v, for a duration of ΔtA long, over the second segmant, twin B approaches earth with the opposite speed