The second lesson, Radioactive Decay: A Sweet Simulation of Half-life, introduces the idea of half-life.
By the end of the 8th grade, students should know that all matter is made up of atoms, which are far too small to see directly through a microscope.
So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.
It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms.