Primary cosmic rays are high-energy particles that originate from space and are mostly protons. When these cosmic rays reach Earth's upper atmosphere, between fifteen and twenty kilometers above the surface, they collide with nitrogen and oxygen atoms, creating unstable particles known as "charged pions." These pions quickly decay into muons, a type of subatomic particle. A muon, which has a very short lifespan of 2.2 microseconds, would not typically travel more than 660 meters, even at speeds close to the speed of light. If created at fifteen kilometers altitude, it would not reach Earth's surface. However, scientists have detected muons on Earth, raising the question of how they manage to reach the ground. This phenomenon is explained by time dilation, a key concept from Einstein's theory of relativity. The principle of relativity, first introduced by Galileo and later expanded by Einstein, suggests that motion is relative and that there is no absolute rest. Time was once thought to be constant for all observers, but Einstein's theory introduced the idea that time is part of a reference frame and can vary depending on an observer's motion. Each person experiences their own time, referred to as "proper time," and this concept is central to understanding relativity. A well-known example of time dilation is the twin paradox. Imagine two twins, Alice and Bob. If Alice travels to Proxima Centauri, a star four light-years from Earth, at 99% the speed of light, Bob, who stays on Earth, would measure her round trip as taking eight years. However, due to time dilation, Alice would experience only about 1.143 years. When she returns, she would be significantly younger than Bob. The situation seems symmetric, but the asymmetry arises when Alice turns back, indicating that she is the one moving, which makes her younger upon return. In the International Space Station, astronaut Sophie Adenot travels at 27,600 kilometers per hour, a speed much greater than typical speeds on Earth. Although the time dilation effect is small, over the course of months in space, she would be six milliseconds younger than someone on Earth. If she had a twin, that twin would be older by six milliseconds. Similarly, muons moving at speeds close to the speed of light experience time dilation. In their reference frame, 2.2 microseconds are enough for them to travel fifteen or twenty kilometers to reach Earth's surface. This was confirmed in 1963 by comparing muon flux measurements at the top of Mount Washington and at sea level. The muon example is a classic demonstration of time dilation, much like the scenario involving Sophie Adenot and the twin paradox.