Hi.  My name is Steve Jones, and I'm going to explain how a water heater works.  Now, in this system, I have a cylinder.  Usually in your house, you'll find a cylinder.  And the cylinder contains a large amount of water, maybe up to 200 liters, depending where you live.  This water has to be heated by some device.  And that is why we have to have a water heater.  Now, a water heater basically is a piece of wire.  And the wire that we use is usually something which has quite a high resistance.  And when you pass electricity through it, it gets hot.  Now the most important thing, of course, is that the electricity doesn't get into the water.  So the actual element of the heater; that's this blue thing here; is a tube.  And the tube, on the outside, is silica.  And inside, you have the coil, which gets hot.  Around this coil, there is a kind of powder, which keeps it intact.  When this gets hot, the outside silica gets hot, and the water gets hot.  But you have to remember that silica is an insulator.  That is, it does not conduct electricity, and therefore, makes the whole thing safe.  Okay, back to the cylinder.  Here is our cylinder.  And we have this device, which we call an immersion heater.  It has a long tube, this tube- silica tube, with an element in it.  Usually there are two of them.  The reason there are two is because maybe you don't want to heat the whole of the water in the cylinder, maybe just half.  Because if you heat the whole lot, then it takes a long time, and it's expensive.  So what happens as you pass the electricity through the wire, which is in the silica, that gets hot and it will heat up the water.  And you will get convection currents within the water, which make the water swirl in this kind of way.  And because hot water rises, the top will be hot while the bottom is cooler.  This is why we have a short one and a long one, because the long one can heat the bottom.  But if we don't have that on, the bottom will remain cold, and the water above it will be hot.  So then you get hot water out of the tap and not cold water.  When you want to take hot water off, you simply open the tap, and the water is pushed out by the cold water coming in at the bottom.  This automatically pushes the hot water out at the top.  And the water will remain hot until it is all used.  Then, of course, you put the heater on, and it will work again.  So that is basically how a water heater system works.
