They are therefore not facts at all, but assumptions. Similar assumptions underpin the use of radioactive isotopes for dating methods. Carbon exists naturally in 3 isotopes. Both 12 C and 13 C are stable atoms. However, this leaves approximately 1 part per million of all carbon atoms, as 14 C, which is radioactive.
Radioactivity is a statistical phenomenon.
This is rather like the chance of throwing a 6 on a die. If you threw 6000 dice, however, you would predict that near enough 1000 would land on a 6. The random event becomes predictable when huge numbers are involved. This is 602 thousand million million million , a number referred to as Avogadro's number, after the scientist who calculated it. The random chance of one 14 C atom decaying therefore becomes predictable.