Big, Bigger, Biggest

This was Batman's project the other day. He gets so frustrated when he can't do something right away and he always wants me to do it.
So we got the marbles out and I let him build a small run. He played with that for 10 minutes and then...
I came back to help him add two more levels. After another 10 minutes...
I helped him add two more levels to make it even bigger.
This kept him busy for a long time!