This tinful of plasticine tops the list of things I have used more or less continuously for the longest time. I bought it in London during a very short visit to England in September 1975. That was 36 years ago*.
I don't think I had a clear purpose in mind when I bought it. And for a long time I didn't do much with it. However, it gradually acquired a unique function in my photographic activities: I now use pieces of it to position snail shells when I'm photographing them.
Originally, each color was a separate stick. After years of handling, the colors are slowly amalgamating. I suspect 35 years from now there will be one mass of indeterminate color.
*The tin is not original; it's an Altoids box.