system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is atomic decay measured in a half-life? Why not just measure it by a full life?

Does it decay fully? Is that why it's measured by half of it decaying?
gpt
Atomic decay is based on the current amount of the substance.  So if you've got 10 lbs. of uranium-238, you will have 5 lbs. in 4.5 billion years.  If you wait another 4.5 billion years, you'll then have 2.5 lbs.  And so forth.

The 'full life' - the time it takes for a substance to completely decay - would be infinite for all substances.