I finally figured it out!

Probably a little over a month ago, Arnold was getting Jackson ready for bed one night and Arnold made a grunting noise.  Jackson then made a similar noise, made two fists, and put them in front of his face.  We thought this was so funny!  We had never seen him do this before and we didn't know where it came from.  At first we thought he was showing us his muscles.  (You know how little kids do that and it is so cute!)  But we had never done that with him before.  Then I was thinking maybe Jackson learned this from watching Arnold laying on the floor on his back with his arms above him fixing our furnace.  He makes all kinds of noises when he's working on things.  Then I thought maybe he learned it from watching Arnold lift heavy things.  Jackson would continue to make this noise and put up his fists every once in a while and I still wasn't sure where it came from, but I thought the furnace thing was my best bet.  So I stopped thinking about it and we just laughed and played every time Jackson would do this.

Today, I was in the kitchen, Jackson was playing on the floor near the kitchen, and Arnold was standing in the living room watching the bears game and all of the sudden I heard a loud, deep "YES!!!" come from Arnold.  I looked up at him and saw his fists clenched and up near his face.  Ah ha!  I got it!  THIS is what Jackson was doing and this is where he got it from!  Arnold did it a few more times during the game, without noticing, and each time, Jackson would do the same.  I got a great photo of the two of them watching the game right after the game winning field goal:

Aren't they cute?!