system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the American Mint keep redesigning our dollar bills with added measures against counterfeiting, when the older bills are left in circulation? Won't counterfeiters just continue to reproduce the older bills?
gpt
Sure. But what happens over time is those older bills slowly come out of circulation as they pass through the banks and are detected and pulled.

No system is perfect - you can't go into someone's wallet from a distance and magically transform all the bills they carry. So this is the best that they can reasonably do without becoming a huge pain in the ass to everyone that carries money.