system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that we put salt in ice cream machines to make the ice colder, but we also put salt on our sidewalks and streets to melt the ice?
gpt
Because you're not making the ice colder in the ice cream machine.  In both cases your goal is to make liquid water colder, but the reasons for wanting colder liquid water are different.  

In the case of the ice cream maker, there's always at least a thin layer of liquid between the cream container and the solid ice, so colder liquid water means more heat transfers out of the ice cream and into the liquid water (so the ice cream freezes fast enough to freeze the air bubbles into the mix which is why ice cream has such great texture).  

In the case of the sidewalks, colder liquid water means more ice melts and flows off the sidewalk and melting continues at lower temperatures, reducing the slipping hazard.