What is wrong about crumbling dry leaves?
I crumbled everything I could crumble as a child, I still like to crumble and eating food with my fingers if possible to crumble it before.
I also like to crumble sand between my fingers and count the grains of sand which are left on my fingers, but nowadays I do not live close to sand, but I did as a child.
There is no harm with crumbling.
Tell your son to crumble his food over a plate in order to not mess the house.
The sensory need of crumbling is met by crumbling.
Another sensation is visual, to see a structure going in to smaller smaller smaller particles.
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English is not my native language, so I will very likely do mistakes in writing or understanding. My edits are due to corrections of mistakes, which I sometimes recognize just after submitting a text.