1, which is mainly caused by the difference of opening shape and air velocity. When blowing, the opening area of the mouth is small and the blowing speed is fast. Because of friction, the blowing gas carries a lot of static cold air nearby and moves together. When it is blown on your hands, the proportion of cold air is large, which will accelerate the evaporation of water or sweat on your hands. Evaporation is an endothermic process, so it will feel cold when blowing. When we open our mouth with our hands, the mouth area is large and the speed of gas is relatively slow. Exhaled gas will not accelerate evaporation when it touches the skin and returns. People feel more hot air, so they think the exhaled gas is hot. Besides, if we blow with our hands close to our mouths, we will basically feel the heat. This is because the air blown out of the mouth is too close to carry the cold air nearby.

2. As we all know, the wind will accelerate the evaporation of sweat on your hands, so the sweat on your hands or the water that hasn't dried after washing your hands will be blown out by gas circulation, and then the water will evaporate quickly. We also know that water evaporation is a process that needs to absorb heat, so it is an endothermic reaction that needs the heat of our bodies to evaporate sweat or water on our hands. When we blow, the speed is not as fast as when we breathe. The faster the speed, the faster the evaporation, so it is cold.