system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it continue to get colder after Dec. 21 (the shortest day of the year)?

As the earth revolves around the sun our days get shorter and longer due to the tilt of the earth with Dec. 21 being the shortest day of the year. After Dec. 24-25 the days begin getting longer again, but the weather continues to get colder until about March. Why does this happen when we are receiving more sunlight?
gpt
It is easiest to understand this in terms of heating days and cooling days.

Imagine if a location on earth gained 1 degree for everyone hour of sunlight, and lost 1 degree for every hour of darkness.

During the fall equinox, it would gain 12 degrees during the day, and lose 12 at night, breaking even.  As the days shortened, it would lose a little more at night, and gain a little less during the day.  

By the winter solstice, with say 9 hours of daylight, you would be losing 6 degrees a day.  Even though the next day would have a few second more light, you would have more cooling time than warming, so it would be colder than the day before.