Thermodynamics is not easy, but it’s not that hard either, it just is. This is a great way to show your friends how pressure and heat are interchangeable. And parents, always remember, for a child, understanding something fundamentally will serve them much longer than learning it by heart.
Ask your friend (or kid) to take a deep breath, and blow it out. Nothing happens. Normal.
Now, this time, ask them to take a deep breath, close their mouth shut, blow out as hard they can into their cheeks for a second without the air escaping (adding pressure), and then to blow it out again. Steam! Visible steam!
Try it. Take a deep breath. Hold your mouth shut, close the passage to your nose, add as much air pressure into the space in your mouth as you can, hold it for a sec, and blow it out. NOTE: Like, a second or two, max, otherwise, you might pass out. I jest, but the pressure is relative to the effort applied by the diaphragm, so it serves no purpose to do it longer than the max pressure that your mouth can handle.
This is what happened. The added pressure heated up the air in your mouth. You blew out the hotter than ambient air, and just like a kettle, the heat escaping your spout heated the air in front of it and made a cloud. (which is why you can see it, like a cloud). (Stay tuned for a lesson in refraction. Eventually. Lol!)
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air.
This science moment is brought to you by Critical Thinking… Critical Thinking: available in fine books everywhere. Like this one.