what comes after twenty-nine?

fu loves to count in the car.. in fact he often goes into a shout
"ONE TWO THREE FOUR...... NINE TEN!"

A few nights ago.. he started his shouting spree again.. I usually try to ignore him hoping by doing so he gets no incentive to shout.. so there he went..

"ONE TWO THREE FOUR........"

However, this time it was different he continued counting until he reached twenty. Delighted as I was, I continued for him..
"twenty-one.." and he continued for me!
"twenty-two, twenty-three, twenty-four.."

then came a pause.. he didnt continue with 25, 26, but later..

"twenty-seven, twenty-eight, twenty-nine.."

and as my eyes grew bigger and bigger.. he counted the last number, the grand-finale..

"TWENTY-TEN".

Boy.. we had a good laugh!