And then I thought about how, for a long time, scientists were puzzled by the fact that the sky is dark at night, even though there are billions of stars in the universe and there must be stars in every direction you look, so that the sky should be full of starlight because there is very little in the way to stop the light reaching earth.
Then they worked out that the universe was expanding, that the stars were all rushing away from one another after the Big Bang, and the further the stars were away from us the faster they were moving, some of them nearly as fast as the speed of light, which was why their light never reached us.
I like this fact. It is something you can work out in your own mind just by looking at the sky above your head at night and thinking without having to ask anyone.
And when the universe has finished exploding all the stars will slow down, like a ball that has been thrown into the air, and they will come to a halt and they will all begin to fall towards the centre of the universe again. And then there will be nothing to stop us seeing all the stars in the world because they will all be moving towards us, gradually faster and faster, and we will know that the world is going to end soon because when we look up into the sky at night there will be no darkness, just the blazing light of billions and billions of stars, all falling.
Except that noone will see this because there will be no people left on the earth to see it. They will probably have become extinct by then. And even if there are people still in existence they will not see it because the light will be so bright and hot that everyone will be burnt to death, even if they live in tunnels.