system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : How and why does ice help swelling?
gpt
Swelling is the bodies response to anything that it perceives as harmful. It is caused by a rush of blood cells and your capillaries opening (called vasodilation) to allow more blood flow to the area for healing. This causes inflammation and heat around the area. 

When you get cold, your blood vessels constrict (vasoconstriction) to keep blood flowing to your vital organs. 

TL;DR: Putting ice on a swollen area helps constrict the blood vessels and keep the blood moving. 👍🏻