Imagine a cat jumping from the top of a corner cupboard, across the room to a free standing bird cage, in such a manner that the motion of the cat is uniform and in a straight line.   If I, the pet dog, observe the flight of the cat as I walk across the dining room floor, I would find that the motion of the cat would be one of different velocity and direction, but that the flight of the cat would still be uniform and in a straight line.

If I could express that phenomenon abstractly ( and I cannot ), I might say:  If a mass ( cat ) is moving uniformly in a straight line with respect to another co-ordinate system ( bird cage ), then it will also be moving uniformly and in a straight line relative to a second co-ordinate system ( me walking ), provided that I am executing a uniform translatory motion with respect to the bird cage.

I, of course, have no idea how I arrived at that conclusion but I seem to be aware of many things, as you have no doubt surmised by now.

To me, relativity has something to do with ‘pick of the litter’, which I, of course, was.  The relativity of the others born to my mother, to me, is insignificant, for I did not pick me.  Someone else did.

Also, in my uncomplicated world, right now, I am hungry and the relativity of my food dish, to me, is some distance across the room.  Thank goodness it’s not moving because my translatory motion may not be uniform and I could, theoretically, miss the dish.

The Wisdom of Willie Bean