I have proof

Someone inherited a habit from her Daddy's family of sticking her tongue out while concentrating.

Her Grandma, Great-Aunt, Aunt, and Cousin all do it.

I tried to find proof of other family concentrating, but this is what I could find:

Cousin Isabelle painting
(A few years ago)

(UPDATE! I found proof that the girls' grandma does it!)

And here's proof that it happens all the time:

While bike-riding

While playing in dirt 

While gluing 

While snacking

While playing baseball

While puzzling

Happy 20 months to my hijita!

It's incredible to imagine that Hazel is almost the same age Ellie was when Hazel was born. 

Hazel-a few hours old
Ellie-20 1/2 months old