Explaining physics to a child is not only challenging, but one needs to be highly imaginative. Physics I feel is more about imagination rather than cumbersome mathematics and equations. Hence it is quite often seen that the best of physicists are not the best of teachers.
Imagine explaining concepts of energy conservation, heat transfer and friction and losses to a small child. These theories have been built over the past century and till date some of the concepts are not very clear. The theories can be taught in the most cumbersome way with a lot of mathematics and quantum mechanics involved or they can be taught in a simplistic way giving a flavor of whats going on to a person who has just started learning the basics of science. I call this the "jiggling theory" of teaching kids.
Imagine we had a bunch of small ping pong balls in a tub and all these balls are constantly jiggling around. Even if two balls bounce against each other they may transfer their jiggling from one ball to the other but the total amount of jiggling remains constant. Now if we have another tub in which there is water and we transfer the ping pong balls to this new tub then although the balls will appear to be jiggling less, the water in the tub will be jiggling more than before thus conserving the jiggling. This is the basic principle behind conservation of energy, the ping pong balls are the particles (could be atoms or molecules) and the jiggling correlates to the kinetic energy of the particles whereas the different tubs (one filled with water and one empty) i.e different environments correspond to the different potentials a particle experiences.
Now imagine two sets of ping pong balls one jiggling more and the other less. Now if we mix these in an empty tub the fast jiggling balls will collide with the slow jiggling ones and will make the slow ones jiggle fast. Now since the total jiggling is constant the fast jiggling ones will slow down. This is the second law of thermodynamics or the basic principle behind heat flow, which says that heat always flows from a hotter object (more jiggling balls) to a colder one (less jiggling balls). Now if we wait long enough all the balls will be jiggling at the same rate, thus giving an equilibrium. In thermodynamics the magnitude of the jiggling of these balls is also known as temperature.
Now if these same balls were placed in a tub filled with water then the water would also start jiggling and the balls would loose some of their jiggleness to the water, thus resulting in losses and giving rise to the concept of friction.
The jiggling theory can be applied to many other fields and can be used to explain many other difficult concepts. All one needs is a little bit of jiggling and whole lot of imagination.
