Because he thought people were taking him for granite.
I've been training as a sculptor for months but I'm not very good at it. Just the other day I made an Elk from limestone which I thought was good, but my art teacher Mr Watson couldn't work out what it was.
I said to him surely he could see it was sedimentary, my deer, Watson.
What did one piece of limestone say to the other?
"I think we need to chalk..."