Bumble's Conveyor Belt

Bumble is my creative one.  She is constantly coming up with "projects" that she thinks would be fun to do.  She asked me the other day if she could borrow the trash bags.  This was a strange request, but I said yes. 
First she lined up the dining room chairs and then taped the trash bags together to form a line...

 

She worked very hard on this...

 

I asked her what she was making and she answered, "A conveyor belt, Mommy!"

 

Then she went on to explain to me that this was a conveyor belt for the mail, and how the mail runs on the belt and gets sorted.  She made letters for everyone and sent them down the belt and they landed in a grocery bag to get delivered. 

Please tell me this kind of "play" is normal!  Where she comes up with these ideas I'll never know!