system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the anti-theft systems at store exits/entrances work?
gpt
There are little tags inside the devices that emit a reply radio signal when they’re subjected to another, higher power radio signal on the correct frequency. 

If you shoplift an item, this tag is still active and the sensors you walk through can detect it. 

When you check out, the cashier runs your item over a device that detects the tag. When the tag is detected, the device emits a very high powered pulse that burns out the tag, deactivating it.  This is the “bong” you hear sometimes.  This pulse does a good job of demagnetizing credit cards, so that’s why there’s signs saying keep them away.