The rays that come from each rock move only so fast. They take time to get from the rock to Dana or Bert. And it is only by when those rays get to Dana or Bert

The rays that come from each rock move only so fast. They take time to get from the rock to Dana or Bert. And it is only by when those rays get to Dana or Bert that they can say if they were at the same time, or one and then the next. Now. Bert is on the bus. It goes, let us say, from west to east on the road. Bert is half way from the west rock to the east rock when each rock hits land -- but he is on the move, and when the rays from the east rock hit his eyes he has gone some way east. The rays from the west rock have not yet got to him. Then, just a tad more time, and the rays from the west rock get to Bert's eyes. But Dana does not move. She sits by the road. The rays from the west rock and the east rock hit her at the same time: each rock is, say, a yard from her, one west and one east. So Dana sits and does not move; she sees the rays from each rock at the same time. Bert is on the bus, and goes from west to east; he sees the rays from the east rock and then the rays from the west rock. And all that is fine. They both know that Dana does not move and Bert does. So they will say that each rock hit at the same time, for Dana (who did not move) saw them hit at the same time. But (and this is the bit that may fry your mind) we can just as well say that Bert does not move and Dana does move! And then we will have to say that the east rock hit and then the west rock! How can that be? We must say, even if you find it odd, that "did the east rock hit, then the west? or the west, then the east? or both at once?" just goes one way for Dana and not that way for Bert. Just like "is this rock to the east or to the west?" may not go the same way for both, if they are not at the same site. (You are not dumb. This is hard. It took Al to work it out!)