I did a weird activity and now I am trying to figure out the physics behind it:
   
While sitting on a chair I kept my feet above the ground and tried to move the chair forward. I was able to. Initially, I thought that perhaps the center of mass won't be changing as I was pushed backwards. But I moved the chair for a while and saw that it had moved a considerable distance and I was still sitting on the chair. Hence the center of mass did change for sure. According to Newton's second law, *there must be an external force acting on the system*(me and the chair). But I am not able to figure out what force is it. I have a feeling that it is somewhere related to the fact that the system is not completely isolated but I am not sure how.   

**Note:**   
I am not in contact with the ground or any other object close to me. I have lifted my feet and have my hands on my lap.