During bathtime, I brush C's teeth while counting to 30; he knows that when I reach 30, we're done. (Ah, the tricks we learn.)
Last night when I reached 30, C perked up and said cheerily, "30, 30, 30!"
I smiled. "Yes, 30!"
He then said, "40, 40, 40, 40!"
Me: "40?"
C: "50, 50, 50, 50, 50...60, 60, 60, 60, 60, 60!"
It took me a minute before I figured out what he was doing. Finally I said, "What about 70?"
C: "70, 70, 70, 70, 70, 70, 70! Seven 70s! Then eight 80s!"
No one has taught him this. It's just one of his many little surprises for us.