Can someone point me toward a good resource for maintaining children's bikes? We got our three-year-old a little twelve-inch two-wheeler (yes, he rides without training wheels!) at a secondhand shop and I'm having trouble with it. You can turn the pedals and make the wheels move, but it has gotten really difficult to do so--there seems to be some kind of invisible resistance. Normally I would just Google around, but I don't know what terms I would use or where to start. Any help?