Minx and Marbles

All my cats have always loved playing with a marble.
Its rolls with next to no touching, it bounces, and it makes a noise on the floor! They love it when I put a marble in the bath as they can chase it round and round for hours without losing it, and it makes a noise!
But I watch them closely as a marble can be easily swallowed or choked on. As soon as they've had enough playing in the bath I put the marble away.
A couple of days ago, Minx found a marble. I have no idea where, but she was playing with it on the lounge floor and I heard the noise- which is very distinctive if you've heard it before. I moved her- with her marble- to one of our bathtubs and she was playing merrily, and a couple of minutes later I heard the marble on the staircase, like it was rolling down. I was a little puzzled, but before I could go and investigate I heard it again- a marble rolling and bouncing down the stairs!
Now I was really puzzled.
And as I was walking to see what was happening I heard it again- a marble rolling and bouncing down our tiled staircase!
When I got there, I was stunned to see Minx pick up the marble she was playing with and carry it back up the stairs in her mouth- where she dropped it and then had a wonderful time chasing and batting it down the stairs again before picking it up and carrying it upstairs again!
I have seen my cats carry their toys around and take them somewhere more entertaining, like around my kitchen stools and step ladder, but none of my cats have ever picked up and carried a marble around!