Does anyone elses dog grind shells?

Tonks enjoys doing this very odd thing. When we go to my OH's mum's house she picks out a nice flat shell from her collection in the garden, lays it flat and sort of grinds it with her front teeth.

If there are any barnicals on there she will strip them off until it is completely smooth. She's not manic with it just does it in a very calm slow way.

When she was staying at my friends house, while we were on holiday, she ground all the blobs of plaster off that had dried on her tiled kitchen floor.

Someone suggested that it might ware her teeth down, but they look fine to me. She does it about twice a week and it can keep her occupied for 30 minutes or so .....which is great for me as i work from home so anything that keeps her occupied is a bonus!! She spends time turning it over in her paws looking for the next flat piece to have a go at.