system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do memory products such as an SD card or flash drive store memory and keep it saved when it doesn't have a power source?
gpt
The controller causes a physical change in the memory module. In floppy disks it was a magnetic change.

SD cards use flash memory which has a set of gates that trap electrons. When electrons are trapped in the gate it changes the resistance of the gate. A new higher voltage is required to discharge the electrons from the gate. By using a voltage greater than the initial charge voltage, but less than the new higher voltage the device can check if a gate was previously charged or not.