system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do glass companies know how to mold an hourglass to have the sand fall precisely?
gpt
They don't need to. They mold the hourglass to have sand fall approximately, add sand, wait the hour, dump the extra out and seal the glass. Its WAY easier to calibrate the sand than to calibrate the neck.