Activity produces extra heat within our bodies. Every cell is like a tiny engine, roaring away. The more you gun those little engines, the more heat you need to "give away" to the environment. Otherwise, you'll get burned by your own motor.

There are 4 ways warm-blooded creatures shed heat. One is by simply radiating heat to the environment. It's only an option when the surrounding air is cooler than body temperature, so after 100 degrees, don't expect help here. Also, the more fur, the more insulation against radiation loss.

The second is by conduction, which means you give the heat away through touch-transfer. Laying in water or on tile will suck away body heat into the cooler material. That's why the spot on the couch where your dog was laying feels warm.

Convection is the cooling method we utilize by turning on a fan. The wind blows over our warm skin and "scrapes away" the surface heat. Taking a dip in the lake also convects heat from your skin to the water. Fur resists convection by trapping water or air close to the skin and warming it, creating a "buffer" layer like a wet suit.

Evaporation is also a very effective coolant. When we sweat, the moisture we put on the surface of our skin changes from liquid to gas, taking heat with it. In addition, lungs and mucus membranes humidify every breath-- when you breathe on a piece of glass, you can see the warm moisture, turned back to liquid momentarily. For dogs, breathing out evaporated heat is their main cooler. They will pant rapidly even when they don't need oxygen/aren't tired. Evaporation also occurs on the surface of their tongue and paws. Dogs with short noses and tongues are at a disadvantage.

Now that you know the mechanics, imagine you are out jogging with your dog (because you're that hip) on a warm California morning. You are sweating, but your running speed creates a breeze that cools you (convection and evaporation). Eventually, you notice that your dog is panting like a machine gun, his tongue appears barely moist and curled up on the end, and he is leaving moist little footprints on the sidewalk that instantly evaporate. You whip out your half-empty Nalgene bottle and offer him water but he seems disinterested. At this point you should--

A. Wait in the shade until the dog's panting slows and he has accepted some water

B. Bathe the dog's underbelly and paws with the remaining water and continue

C. Fan the dog with a piece of cardboard while you walk slowly towards home-- and the AC

Life is full of these sorts of dilemas, where you must pick the best of several possible solutions. In the case of having limited water, I would make hydrating the dog a priority and choose Option A. Stop activity, get out of the sun, and have the dog lay on the coolest surface available to conduct heat off. Continue with great caution. Once a dog (or person) has experienced a heatstroke incident, they will more prone to heat distress the rest of their lives.

Let's continue our journey. Concerned that your dog may be overheated, you lead him gently towards:

A. The nearest brackish ditch, and encourage him to splash in it, neck-deep, for a few minutes.

B. A hose next to the nearest house, wet him down quickly and move on before someone sees you

C. The nearest ditch, where you scoop up water in a cast-off Starbucks cup and pour it directly on his head and back

Option B is stellar, provided you can do it discreetly or with permission. However, it's not effective to just briefly splash the dog: douse him for several minutes all over. A furry dog is not likely to over-cool but if you are using this technique on a person or a short-haired dog, stop before they start shivering. Second to running water, immersion in still water is also great for cooling. So long as the water is less than 100 degrees (the dog's body temperature), conduction will occur immediately. Keep the dog moving to "scrape off" heat through convection. If the water is not deep enough to cover most of the dog, focus water on areas of skin, not fur. Flick water up on the belly, groin and inside of the legs. If he has stand-up ears, brush some moisture there, too, avoiding flooding the ear canal.

When your strategy involves small amounts of water on the underbelly for direct evaporation, keep in mind that warm water works better than cold for this purpose. Giving your dog icy-cold water to drink is also not necessary. It will be absorbed and put to use faster if it is room-temperature.

Hydration begins the day before exercise. If your dog isn't much of a drinker, try flavoring the water or floating treats in it. Soak the dog's kibble in extra water for a flavored stew.

Shaving a long-haired dog in summer is of disputed value. A dog's insulating coat works both ways: especially over 100 degrees or in sunlight, the fur will deflect extra heat away from being absorbed through the skin. In any case, keeping a furry dog brushed out of the soft undercoat will let more air or water circulate close to the skin for convection.

Now that you know how heat is shed, you can evaluate whether your dog-- given the activity level you expect-- should have a cooling vest, and which type. Don't listen to what manufacturers say about why their product is the best. For instance, most water vests are incorrectly advertised as cooling via evaporation and equated with human sweating. Not quite. The dog's heat first has to conduct to the wet vest through contact. The vest then cools itself to clamminess by convection and evaporation.

"What does it matter?" you exclaim. "It's all heat lost, one way or another!" It matters because, certain areas of most dogs are highly resistant to the mechanism of conduction. The furry dog in the photo to the left, with a vest on its back, is getting very little benefit. It is a vest cut for fashion, not efficiency. Place your hand on your dog's side/back. Warm? Now place your hand on your dog's belly. Is there a difference in the amount of heat conducted to your hand? Put the cooling where the heat is, duh. Trying to conduct heat through a fur barrier is dumb. A short-haired dog like the one on the right can conduct to this vest.

On the right is a sensibly-designed cooling vest. It is centered over the least-furry area of the dog, where conduction occurs most efficiently. There's not a bunch of pretty, excess fabric to mat down the dog's fur and stifle convection. This is a working-quality cooling vest.

Wherever they are placed, vests are just a fancy piece of material to hold some sort of heat-absorbing liquid. This coolant may be water or a magical "phase-change material" or "state of the art bio-based cooling packs". Wow... water sounds so mundane and the others so cutting-edge and neat-o. Here's an industry secret.... tap-water is a bio-based phase-change material too! You can refer to the ice in your drink as "solid-state dihydrogen monoxide" (H2O) if you'd like.

Simply put, anything touching the dog, that is cooler than the dog, will conduct heat away from the dog. The various coolants differ in the temperature at which they become solid, liquid or gas, and vary slightly in conductivity. In the case of frozen water, it may get a little too cold and burn or numb the skin. Your criteria for selecting a heat-absorbing vector can center on how safe it is in the event it is devoured by a curious dog or child.