In our discussions on gravity, I thought it might be a good moment to introduce Bobby to the concept of the 'equation' in science. I asked him if he could come up with an equation and without hesitation he said E = m c^2. I spent a few minutes explaining the different terms in this equation, i.e. that 'c' is a constant representing the speed of light, and this number never changes; that 'm' stands for mass, and this is the input (as an important aside, Bobby was fully aware that 'mass' and 'weight' are two different things, the latter being the effect of gravity on mass); and the 'E' is for energy. We then moved onto the four fundamental forces of nature (electromagnetic, strong and weak nuclear forces, and gravity) - the first three act at short range, while gravity is a long-range force. After I introduced the equation for the force of gravity to Bobby - F = G (m1 * m2) / d^2 - and explained to him what the terms are (G the gravitational constant, m1 and m2 the two masses upon which the force acts, and d the distance between the two masses) I showed him what happens to the force if either or both masses increase (the force increases). I then asked Bobby what happens to the force if I increase the distance between the two bodies, and without hesitation he replied it decreases. Spot on!