Thanks All! The bike is a single speed Dahon Boardwalk folding bike with coaster brakes and hand brakes. We got it in June and I had in tuned in the beginning of August. I am not sure if it has a derailleur but I'll take a look today. Maybe I should take it back to the LBS where I had it tuned. We plan on riding 12 miles again over the weekend and I don't want her chain to fall off while she is riding...