I understood that Bobby needed to go over certain concepts before next week's test, and one of them was seasons on Earth. Using the globe that is in his room, I showed how the tilt in the Earth's axis affects the amount of sunlight that impinges upon the surface, and in turn this determines whether the northern or southern hemisphere experiences winter (and the other hemisphere by definition summer). I started by reminding him that all planets not only orbit the sun, but also rotate about their own axes. All planets but one have an (arbitrary definition) up-down axis and when I asked Bobby which one did not he replied without hesitation Uranus (Bobby has a great memory). We then watched a short documentary on the bolide that crashed into the Russian steppes a few years back that was caught on several dash-cams. The size of that rock was some 17 yards across, not large enough to cause widespread damage, but sufficiently large to be spectacular in the sky and create a shock wave strong enough to knock people off their feet. This bolide is a larger version of what we know as 'shooting stars', which are in effect small grains dust that burn up in the atmosphere.