To force the water through the coffee, the simplest espresso machines use pressure that comes from heating water inside a sealed vessel. These types of machines can be bought for around $50, and there's even one that is made especially to take on a camping trip. They all work on the same principle, so we'll take a look at one of the camping-style machines.
In this type of machine, the coffee is packed into a funnel-shaped piece of metal that has a tube extending to the bottom of the reservoir. A few ounces of water are put into the reservoir and the top is screwed on.
When the water is heated over a fire, pressure builds inside the vessel, and the only way for it to escape is up the tube, through the coffee and out of the tube in the top. Since the end of the tube is under water, the pressure forces the hot water up through the tube.
There are some disadvantages to a machine like this. The pressure in the system depends on the temperature of the water. The temperature required to build up enough pressure to force the water through the coffee might exceed the ideal brewing temperature.
This is why some home machines incorporate a pump. Let's take a look at one of these.