system
Given the following post, respond as if you're commenting on Reddit:
human
If the universe has been around for 13.7 billion years, why does anything still have radioactivity?

We often use radio active dating to predict the age of objects, how can things get "recharged" with radio activity why hasn't it all decayed over the billions of years? 

Thanks.
gpt
Radioactive elements are constantly being produced in stars or 'exploding' stars. In fact, the big bang produced almost only hydrogen and helium, all the rest of the elements have been created in stars through fusion since and still are being created. But not all radioactive elements have to originate in stars.

Carbon dating for example can be used to determine the age of once living objects for about 50,000 years into the past. It determines the ratio of inactive Carbon ( C^12 ) to radioactive Carbon ( C^14 ). C^14 is constantly being created in the upper atmosphere, as cosmic radiation produces neutrons, which hit Nitrogen atoms ( N^14 ) and create C^14 atoms. C^14 to C^12 ratio in the atmosphere has been almost constant for most of recent history because of this. Now, a living plant organism uses C from the atmosphere to grow, depositing it in its tissue, and animals will either directly eat plants or eat animals that have eaten plants. This keeps their C^12 to C^14 ratio in their tissues at a level that corresponds to the current concentration in the atmosphere. When they die, no more C enters their body and the concentration stays fixed. C^14 will decay though, half the atoms of C^14 will revert back to N^14 in about 5700 years. So the longer an organism has been dead, the less C^14 it will have. By measuring the concentration of C^14 and C^12, you can determine its age.