As someone who spent many midlife years running in summer triathlons and who enjoys a physically demanding self-reliant rural life, I count heat and cold among my most important first-aid remedies.

Strained muscles, Achilles tendinitis, tennis elbow (I got mine from a day of tossing cordwood off a wood-splitting machine), big bruises, bumps on the head—I’ve known ‘em all. Advancing age has brought painful arthritis in knees, neck, back, elbows, wrists, thumbs, and fingers.

But when do you use heat and when to you use cold? Let’s review which is best—and learn how to make an easy cold pack and heat pack, too.

When to Ice (Cold Therapy)

Use cold to treat an acute injury with swelling—sprains, bumps, bruises, tendinitis. Cold numbs the nerves and constricts the blood vessels, reducing swelling and tissue breakdown and easing pain. (It also works for temporarily reducing under-eye puffiness.)

→ Get more natural remedies for bruises.

The time-honored cold pack for a sprain or a fresh, swollen bruise is a bag of frozen corn or peas. The bag conforms to the injured part and treats the injury—then you eat the thawed vegetables for supper.

In winter, I’ve stuffed a plastic bag with snow. I’ve also frozen water in a paper cup, torn back the top of the cup, and massaged the injured part with the ice. Slow, circular massaging movements help to prevent frostbite that can occur from continuous application.

You can make your own reusable cold packs in a jiffy: Just fill a long, 100 percent cotton sock with rice, small beans, flax seed, dried corn kernels, or other hard seed or grain produce, leaving enough space to tie a knot. Keep a couple on hand in the freezer.

Endurance athletes sometimes hasten recovery by immersing their entire bodies in an ice bath.