 So let's start with a bit of revision. So here's space, there's the Earth, and it's atmosphere, and here's the Sun. Now cosmic rays from the Sun cause these particles that we call muons to form in the upper atmosphere. Let's look at things from the Earth's frame of reference. So the muons are traveling at some speed v, and the atmosphere has some thickness l. Now we detect these muons on the surface of the Earth. Now the time it should take the muon to reach the Earth is just l over v. But it turns out that these muons should decay very, very quickly in some time tau that is actually much smaller than l on v. So how can we still detect these muons on the Earth if they should all decay long before they reach the surface?