A muon has a lifespan of 2.2 milliseconds. Traveling at the speed of light (which is impossible for anything with mass) the maximum distance a muon can travel is 660 meters. So if a muon is created by cosmic rays bombarding our upper atmosphere, 10km up, how can we see a muon if we build a cloud chamber here on the surface?
The answer is fucking brilliant. Einstein knew. Time is relative. The faster one travels, the slower time moves. Google it.