Hank: Probably one of the first things you do when you bang your knee or twist your ankle is put an ice pack on the injury. You probably know that that can help reduce swelling and dull the pain, especially within the first couple days after you get hurt. But why interfere with the way your body’s trying to naturally heal itself?

When you pull a muscle, stretch a tendon, or burst a blood vessel, that triggers your body’s inflammatory response: it sends a bunch of blood cells and fluids to repair the damage. And that is an important part of the healing process.

The problem is, your inflammatory response tends to overreact. If you’ve ever gotten hay fever in the spring and felt like your eyes and nose were going to melt off your face, you know what I’m talkin’ about. Too much swelling can cause a bit of a traffic jam with all the fluids rushing in, which can cut off nearby cells from getting the oxygen they need to keep functioning.

And that is where icing an injury, also known as cryotherapy, can help. When you put ice on a part of your body that’s inflamed, the tissues and blood vessels contract, limiting how much blood and other fluids can get to the area and bringing down the swelling. Cold can also slow down the cells’ metabolism, so they use the oxygen they do have more slowly and don’t die. If it’s at a joint, like if you sprain your ankle, you’ll be able to use it a little more, and maybe do some rehab exercises.

But if you leave an ice pack on for too long, it can actually do more harm than good. If the injury site gets too cold, your body will flood in even more blood cells and fluids, trying to warm it back up, even if you’re still using the ice pack. This renewed blood flow is called hyperemia, and it’s why doctors say it’s generally good to limit cryotherapy to about 10 or 20 minutes at a time. After that, let your injury warm up for 10 minutes or so before putting the ice pack back on.

Ice can also reduce the pain you feel. When they get cold, the ions that carry messages around between neurons have a harder time getting around, reducing the pain signals going to your brain. Also reducing lots of other kinds of signals, which is what numbness is.

So when you get hurt, it is important to let your body do its thing and heal. Ice just helps make sure your body doesn’t overreact, and keeps you a little more comfortable in the process.

Thanks to all of our Patreon patrons who keep these answers coming, and for more on the science of injuries, check out this other SciShow video where Michael explains why papercuts hurt so dang much.