Out of Gas

I took Nathan out to ride his bike on night after dinner. Our neighborhood is very hilly so we go to a section of the neighborhood that is almost flat. Nathan climbed on and started. I gave him a little push but he wasn't able to take off. We did this a couple more times and he still wasn't able to take off as we were still at a slight incline. Nathan turned around and he stated, "Mom, I think we are out of gas."