How can a small pouch heat up or chill on command?

A pouch can change temperature on command because it holds a chemical change waiting for a trigger. For warmth, one common kind is a sealed packet of iron powder. When you open it, the air reaches the iron, and oxygen and water react with the iron to form rust. That oxidation is exothermic, which means it gives off heat. Salt is usually added to speed it up. The packet reaches its maximum temperature in about 20 minutes and keeps giving a modest level of heat for many hours. In other words, a hand warmer is a rusting machine that has been tuned to be useful.

Another kind can be used again and again. It holds a solution that has been heated and then cooled without forming crystals, which makes it supersaturated. Flex a small metal disk inside and you create nucleation centres, places where crystals can start. Crystallisation spreads through the pouch, and the heat that was needed to dissolve the salt is released again. Dip it in very hot water and the cycle can restart.

A cold pack plays the opposite game. An instant cold pack is a bag of water inside another bag holding a solid such as ammonium nitrate or urea. Squeeze it and the inner bag breaks, so the solid dissolves in an endothermic process. That means dissolving takes in heat, which it draws from its surroundings, and the pack gets cold quickly.

So one pouch gives out heat and another takes it in, but both rest on the same idea, a stored change that moves heat, running in opposite directions. Be careful with the warm kind: it can reach 74 °C, so it should not be put against the skin of babies or young children.