To get ready for the science expo, I wired the timer to the track in a semi-permanent way, then I helped Liam run a series of timing runs on the track. This was my opportunity to walk through the scientific method with him. It was a bit tricky to do it in a way that a second grader can understand. For instance, although Liam is in an advanced math program, he is only now starting to work on the concepts of fractions. Decimals are not even on the horizon yet. So I needed to figure out a way that he could report all of the results in whole numbers. This did require me to explain the idea of milliseconds, but he got that readily enough. So we measured the mass of the car in grams, and the times in milliseconds. He had no trouble seeing 1.042 seconds and writing down 1042 ms.
We did five runs of the car at each mass. For the mass variable, I mounted a small box lid on the top of his car. Luckily, his car design has a flat top. This way, we were able to change the mass of the vehicle without significantly changing the aerodynamic cross section of the car as it travels down the track. I didn't expect him to understand the various complexities of what is actually going on to determine the time of the car, from rolling friction to air resistance to potential and kinetic energy. Nor did we go into Galileo and the Newtonian laws of motion. Keeping his attention on this for even 20 minutes at a time over several sessions was hard enough. I just asked him for his hypothesis. Would a light car go faster or slower than a heavier car? He thought a lighter car would go faster. So that's where we started.