system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - If radiation is like small particles shooting outward and through objects, how do objects stay radioactive for long amounts of time?

How is it that objects can become radioactive, if they weren't already? And how come radioactivity lingers for so long?
gpt
radioactive particles are released when unstable atoms decay into a more stable form.  this process is random but has a measurable rate, so e.g.  uranium-235 has a half life of 700 million years, so if you have a mass of that material, after 700 million years about half of the atoms will have decayed into thorium-231 (which itself has a half life of around 25 years)

as for why it can steadily put out radiation and last that long, there's a lot of atoms in a chunk of something.

as for becoming radioactive, when neutrons are released by radiation, they can be captured by other atoms, changing the number of neutrons in the nucleus and making it unstable.  unstable atoms decay, and that process releases radiation.

alternatively, and more likely what you're thinking of, in the case of stuff like chernobyl:  the dust and ash contained radioactive material from the reactors, and the dust got caught in  people's clothing, hair, etc. and continued to give off radiation. that was why they had to discard all the clothes and protective equipment from people who were in the area.