Twin paradox: One of the most well-known "paradoxes" in history, predicted by Sir Einstein's special theory of relativity. It takes two twins, born on similar date on Earth. One, Albert, leaves home for a trip about the Universe at very high speeds (very close up to that of light), whereas the other, Henrik, remains at home at rests. The special relativity predicts that whenever Albert returns, he will discover himself much younger than the Henrik.
That is really not the paradox. The paradox stems from challenging to naively examine the condition to figure out why. From Henrik's point of view (and from everybody else on Earth), the Albert appears to speed off for an elongated time, linger around, and then come back. Therefore he must be the younger one, which is what we observe. However from Albert's point of view, it's Henrik (and the entire of the Earth) which are travelling, not he. According to the special relativity, when Henrik is moving associative to Albert, then Albert must compute his clock as ticking slower -- and therefore Henrik is the one who must be younger. However this is not what occurs.
So what is wrong with our investigation? The key point here is that the regularity was broken. Albert did somewhat that Henrik did not – the Albert accelerated in turning around. Henrik did no accelerating, as he and every one the other people on the Earth can attest to (neglect gravity). Therefore Albert broke the symmetry, and whenever he returns, he is the younger one.