For many centuries scientists suspected that everything is made from atoms.
Most atoms are stable, but some are unstable.
Atoms that are unstable can decay through a process called fission; fusion is the reverse process.
The radioactive decay of a single atom is random, but the decay of large numbers of atoms is predictable.
This allows us to use radioactive decay to find out how old objects are.
The radiation released by radioactive decay can also be put to other uses.
Radiation is energy that travels. Some radiation surrounds us all the time both from natural and artificial causes.
More information is available in Radiation Basics on the Australian Radiation Protection and Nuclear Safety Agency website.