When you experience swelling and pain, you’re in the acute phase of an injury. But “by reducing the swelling, there’ll be less pressure on the pain receptors, and that will make the pain hurt less,” says Eric Sampson, MPT, the physical therapy director at Spine and Sports Medicine in New York City. In a Howcast video online, Sampson explains that ice reduces the swelling around the tissue or joint area and moves you onto the next stages of injury recovery.

During this rehabilitative stage, heat can soothe and relax muscles that are stiff and sore. “Heat promotes blood to the area,” Sampson says, “so it can help with the last phase of your healing.”

But depending on how bad the injury is, don’t use heat for the first three days after getting hurt, Sampson says.

Also, use ice or heat only in 15- to 20-minute intervals. And never place it directly on the skin.