Thanks. Problem is, like I said, how to say it in a few sentences and at primary school level. My target market is the old model c primary schools. I work at high schools as well, but it is quite difficult to get an appointment there.
Well Let me have a crack at it. (So we talking about a paragraph here?)
Between 1905 and 1915 Albert Einstein developed the theory of General relativity. In General relativity Space and time can be thought as one thing and that Mass acts upon it.
Think of a bed sheet being stretched out as space-time and a bowling ball lying upon it, bending the space-time around it. Why is it theory useful? It gives us a clue to how mass and gravity work in space times, and helped us learn that there are things like black holes.
The second theory of Albert Einstein is special relativity, and they key to it is understanding that no matter how fast you are moving you will always measure the speed of light at the same speed, and for this to be possible it means that the faster you are moving the slower your measurement of time is going to be. It may sound strange but with out General Relativity and special relativity things like GPS won't work that is because the satellites that give us GPS work on clocks. And the clocks on the satellites run at different speeds that the ones here on earth and we have to keep adjusting the clocks on these satellites so that GPS works properly.